py-coverage: updated to 5.0.1

Version 5.0.1:

- If a 4.x data file is the cause of a "file is not a database" error, then use
  a more specific error message, "Looks like a coverage 4.x data file, are you
  mixing versions of coverage?"  Helps diagnose the problems described in
  `issue 886`_.

- Measurement contexts and relative file names didn't work together, as
  reported in `issue 899`_ and `issue 900`_.  This is now fixed, thanks to
  David Szotten.

- When using ``coverage run --concurrency=multiprocessing``, all data files
  should be named with parallel-ready suffixes.  5.0 mistakenly named the main
  process' file with no suffix when using ``--append``.  This is now fixed,
  closing `issue 880`_.

- Fixed a problem on Windows when the current directory is changed to a
  different drive (`issue 895`_).  Thanks, Olivier Grisel.

- Updated Python 3.9 support to 3.9a2.

(adam)