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* add official Python 3.9 support
* improve many docstrings
* add DeprecationWarning for `bitdiff()`
* add DeprecationWarning when trying to extend bitarrays
from bytes on Python 3 (`bitarray(b'011')` and `.extend(b'110')`)
* C-level:
- Rewrote `.fromfile()` and `.tofile()` implementation,
such that now the same code is used for Python 2 and 3.
The new implementation is more memory efficient on
Python 3.
- use `memcmp()` in `richcompare()` to shortcut EQ/NE, when
comparing two very large bitarrays for equality the
speedup can easily be 100x
- simplify how unpacking is handled
* add more tests