Text2qti

* Updated `.bat` file for generating Windows GUI executable to account for
`pyproject.toml` and recent Python versions.

* Switched packaging to `pyproject.toml`.

* Updated dependencies: Python Markdown >=3.4.4 is now required to avoid HTML
escaping bugs in prior 3.4.x releases. Require latest bespon (>=0.7.0).

* Fixed bug that prevented indentation starting with a tab from being parsed
correctly (68).

* Improved README explanation of indentation in the text2qti format (59).

* Setting a custom LaTeX rendering URL should typically not be necessary with
recent versions of Canvas. The LaTeX rendering URL now defaults to
`/equation_images/`. Improved config file handling related to LaTeX
rendering URL. Updated LaTeX img template for recent Canvas. LaTeX URLs
now escape the slash character `/` in equations.

* Numbers in scientific notation with leading zeros in the exponent are now
permitted for siunitx notation within Markdown. The zeros are stripped
during processing.

* Leading plus signs `+` are now stripped for siunitx notation within
Markdown.

* Added `amsmath`, `amssymb`, and `siunitx` to template for LaTeX solutions
export.

* Fixed a bug with error handling when Pandoc fails to export solutions (47).

* For question groups, the value of `pick` is now allowed to equal the total
number of questions (44).

* Added support for Pandoc-style attributes on images:
`{id .class1 .class2 width=10em height=5em}` (41).

* Executable code blocks now use PATH to locate executables under Windows, and
thus now work with Python environments. Previously PATH was ignored under
Windows due to the implementation details of Python's `subprocess.Popen()`
(34, 42).

* Added command-line options `--solutions` and `--only-solutions` to
command-line application. These generate solutions in Pandoc Markdown,
PDF, and HTML formats. Pandoc Markdown solutions are only suitable for
use with LaTeX and HTML (35).

* Added quiz-level options `feedback is solution`, `solutions sample groups`,
and `solutions randomize groups` for customizing solutions. Added
group-level option `solutions pick` for customizing solutions. Added
question-level syntax involving `!` for providing solutions.

* In quiz parsing, simplified question type handling and error checking.

* Fixed bug in calculation of total points possible per quiz.

* Added `text2qti_tk` executable, which provides a basic graphical user
interface (GUI) via `tkinter`. Added build scripts in `make_gui_exe/` for
creating a standalone GUI executable under Windows with PyInstaller (27).
* In executable code blocks, `.python` now invokes `python3` on systems where
`python` is equivalent to `python2` as well as on systems that lack a
`python` executable. The README now suggests using `.python3` and
`.python2` to be more explicit (22).
* Added a new keyword argument `executable` for executable code blocks, which
allows a custom executable (including path) to be specified for running
code. Added support for periods in code block language attributes, so that
things like `.python3.8` are now possible.
* Fixed bug caused by swapped identifiers in QTI XML (18, 19). Now quiz
descriptions work with Canvas and question titles appear in Canvas in the
quiz editor (but not in the student view). The following options now work
with Canvas: `Shuffle answers`, `Show correct answers`,
`One question at a time`, and `Can't go back`.
* Installation (`setup.py`) now requires `markdown` >= 3.2 to ensure
compatibility and avoid Markdown parsing bugs fixed in 3.2 (31).
* Fixed bug that produced incorrect QTI output paths when using quiz files
outside the current working directory (28, 29).
* README now covers more options for installing the development version (20).

* Improved preprocessing for siunitx notation, LaTeX math, and HTML comments.
Fixed catastrophic backtracking in LaTeX math regex (11). Added support
for newlines in HTML comments. The preprocessor now skips backslash
escapes, inline code containing newlines, and fenced code blocks (as long as
they do not start on the same line as a list marker). The preprocessor now
handles the escape `\$` itself, since Python-Markdown ignores it (14).
* Python-Markdown's Markdown-in-HTML extension is now enabled (13).
* Added quiz options `Shuffle answers`, `Show correct answers`,
`One question at a time`, and `Can't go back` (10). These options are
apparently ignored by Canvas, but may work with some other systems.
* Revised README to clarify that some features are apparently not supported
by Canvas (16).

* Added support for multiple-answers questions.
* Added support for short-answer (fill-in-the-blank) questions.
* Added support for file-upload questions.
* Added support for setting question titles and point values (9).
* Added support for text regions outside questions.
* Added `--pandoc-mathml` command-line option. This converts LaTeX to MathML
via Pandoc, rather than using a Canvas LaTeX rendering URL (4).
* Added support for comments at the top level of quiz files (outside Markdown
content like questions, choices, or feedback). HTML comments within
Markdown are now stripped and no longer appear in the final QTI file (2).
* For numerical questions, exact answers with allowed margin are now treated
as exact answers, rather than being converted into ranges of values. This
primarily affects how feedback for incorrect answers is worded (7).
* Essay questions now support general feedback.
* Fixed a bug that prevented incorrect question feedback from working.
* Relaxed indentation requirements for quiz titles spanning multiple lines.
Indentation for wrapped lines must now be at least 2 spaces or 1 tab, rather
than being equivalent to that of the first character in the title.
* Fixed a bug that allowed trailing whitespace to cause incorrect indentation
calculations, resulting in indentation errors in valid quizzes.