Instead of leaving abstract methods without definition when running on
the other platform (Unix vs Windows), define them and raise a
PlatformMismatchError if called on the wrong platform.

This fixes pylint warnings such as
   Abstract class 'WindowsFileLock' with abstract methods instantiated
   Abstract class 'UnixFileLock' with abstract methods instantiated

Fixes tox-dev#102

Instead of defining a custom exception, PlatformMismatchError, use the
standard NotImplementedError.

Add tests of UnixFileLock on Windows systems and WindowsFileLock on
non-Windows systems, to make sure they are not abstract (but still
raise NotImplementedError). This avoids pylint errors in code using
filelock.

Also added a note to the change log.

…rror

for more information, see https://pre-commit.ci

The Python standard library module "inspect" has a function called
"isabstract". That is not a spelling mistake. Silence flake8 SC200
warning about "isabstract".

Make a single test, and select lock_type based on sys.platform.

Next minor release, and today's release date in the change log.

To make it a bit clearer, move the preliminary check of abstractness
before the instantiation of a lock, so that the creation of the lock
and the pytest.raises(NotImplementedError) are adjacent.

Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>