Txt2tags

* Raise minimum supported Python version to 3.7 (Jendrik Seipp).
* Add continuous integration tests for Python 3.10 and 3.11 (Jendrik Seipp).
* Add ConTeXt target (224, goetzschnell).
* Fix W3C validator warnings (225, goetzschnell).
* Don't use \multicolumn for single LaTeX table cells (Jendrik Seipp).
* Fix LaTeX escapes (244, ThexXTURBOXx).
* Use correct table title separators for Markdown target (243, db-inf).

* Make CSS style prettier (Jendrik Seipp).
* Remove `--toc-only` and `--toc-level` options (Jendrik Seipp).
* Remove `--mask-email` option (Jendrik Seipp).
* Polish help text (Jendrik Seipp).

* Escape underscores in Latex headers (Jendrik Seipp).
* Don't center Latex tables (Jendrik Seipp).
* Use booktabs package for prettier Latex tables (Jendrik Seipp).
* Use normal font for Latex table headers (Jendrik Seipp).
* Remove `--dump-config` option (Jendrik Seipp).
* Remove `--dump-source` option (Jendrik Seipp).
* Remove txt2tags version from output. (Jendrik Seipp).
* Remove support for multiple input files (Jendrik Seipp).

* Add Markdown target (Eric Forgeot, 213).
* Drop `--encoding` option and always use UTF-8. Files in other encodings are not supported anymore (Jendrik Seipp, Aurelio Jargas, 221).
* Show the original error message when a file read/write operation fails (Aurelio Jargas, 216 217).
* Improve the exception handling when reading input data from files and from stdin (Aurelio Jargas, 218 219).