Literary

This release moves to a namespace package, in order to support `literary-build` installing under the same namespace

This release removes the configurations system based upon various standardised Python files. Instead, it now uses the traitlets configuration system. This means that some commandline behaviour has changed, and configuration is no longer read from the aforementioned files.

Configuration should now be specified in a `literary_config.[json,py]` file placed in one of the `jupyter` search paths, or at the root of the current project. It is assumed that, when running command-line programs, the working directory _is_ the root of the project.

This now means that the same configuration system is used at runtime as well as by the command-line.

This release provides better integration with nbconvert, namely:
- registering the exporter under an entry_hook
- installing template to share/jupyter/nbconvert/templates/literary
- removing hardcoded export tag from template
- renaming preprocessor to `LiteraryTagAllowListPreprocessor`

It also improves support for empty configuration files, and the use of a `.literary-project` marker

Add `set_except_hook` flag to `install_hook` which installs the `traceback.print_exception` excepthook.

Notable commits:
a10e563 Bugfix: restore conditional dependency for astunparse
3e86ff2 Feature: raise Exception / warn when exported code has `get_ipython` calls
7c15029 Bugfix: fix 4 (patching property descriptors)
6639ae4 Bugfix: raise informative error when subcommand
d7cae0c Feature: explicitly load config from config file
bbde9ea Bugfix: don't remove the cwd from sys-path This is a useful feature for notebooks which are not members of the package

Notable commits:
fb34f73 Feature: support ast.unparse in Python 3.9
6639ae4 Bugfix: raise informative error when subcommand omitted
7c15029 Bugfix: fix 4 (patching property descriptors)
3e86ff2 Feature: raise Exception / warn when exported code has `get_ipython` calls
c7a6638 Feature: add IPython dependency