Some errors are platform-specific (e.g. using asyncio.ProactorEventLoop
only exists on Windows and thus mypy complains on Linux) so ignoring those
is needed on Linux but not on Windows. Yay.

This error is raised when the parent directory of a write check location
doesn't exist.

On Windows, the loop was recreated on every call to `construct_asyncio_loop()`,
whereas on Linux the existing loop would be reused if possible. Now both
platforms work the same.

The new values work on Windows too.

This allows the temp directory to be cleaned up on Windows.

On Windows Python just hangs because of a misinterpretation of the
returned error. See https://bugs.python.org/issue22107

Otherwise Windows doesn't allow deletion.

Globbing is the only way in which we can convert arbitrary frame sequences
to a video; the other input options all assume that the frame numbers are
sequential, and stop at the first gap. Globbing just skips gaps and
actually uses all available frames.

The workaround consist of doing the globbing in Python and creating an
index file that lists all the input files.

This is required for pytest coverage reports, and silently fails if not
available.

We should always pass POSIX-style paths to the CLI, it makes life much
easier.

Pathlib2 was used to support Python 3.5, but we're not using that version
any more anyway.

The 'blender_render' and 'blender_render_progressive' commands now look for
a file named 'thefile-overrides.py' in the same directory as
'thefile.blend'. If it exists, Blender will be told to load that Python
file between loading the blend file and starting the rendering.