Ankipandas

Removed

- `Collection.write` now raises a `NotImplementedError` because
https://github.com/klieret/AnkiPandas/issues/137 has so far not been
resolved and has caused numerous problems.

Fixed

- Ensured compatibility with pandas 2.0

Fixed

- Require pandas < 2.0 for now because of API changes in pandas 2.0.
See 143

Fixed

- Fix future warning about `Index.__and__`

Removed

- Python 3.6 has reached its end of life and is no longer supported. This is to
ensure that we can keep the code base modern and developers happy.

Fixed

- When updating cards or notes, certain SQL search indices that are usually
automatically created by Anki could be left missing. While these do not
change any of the information in the database and can be created trivially,
this caused Anki to refuse to load the database.

Documentation and developer happiness

- Various improvements to the documentation
- Work on modernizing the code base

Fixed

- The SQLite connection to the database is now always closed right away by the Collection.
This solves issues where the connection was kept open even though all collection and
AnkiDataFrame objects were deleted (there is additional garbage collection trouble).
Read more in [issue 75](https://github.com/klieret/AnkiPandas/issues/75)