Lib3to6

- Compatibility fixes for Python 3.8
- Add support for f-string `=` specifier
- Add support for `:=` walrus operator (except inside comprehensions)

- Fix python 2 builtins were not always overridden correctly.
- Fix pypy compatibility testing
- Better mypy coverage

- Recursively apply some fixers.

- Move to gitlab.com
- Use bootstrapit
- Fix bugs based on use with pycalver

- CheckErrors include log line numbers
- Transpile errors now include filenames
- Added fixers for renamed modules, e.g.
.. code-block:: diff

- import queue
+ try:
+ import queue
+ except ImportError:
+ import Queue as queue

- Better handling of package_dir
- Change to `CalVer Versioning <https://calver.org/>`_
- Remove console script in favour of simple ``python -m lib3to6``
- Rename from ``three2six`` -> ``lib3to6``