Use platformdirs instead of appdirs

[felixonmars]

Type of changes

• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

Use the better maintained platformdirs instead of appdirs.

[althonos]

Hi @felixonmars ,

In the current state, I'll have to reject this pull request. platformdirs ~=2.0.2 only supports Python 3, while we try to maintain Python 2.7 compatibilty before the next major release. Maybe try to use conditional specifier for Python2.7 using an older version, if possible?

[coveralls]

Coverage remained the same at 95.235% when pulling 15743aa on felixonmars:platformdirs into 196b5f5 on PyFilesystem:master.

[felixonmars]

platformdirs ~=2.0.2 only supports Python 3

This is not true. Their Python 3 only version is not yet released, 2.0.2 on PyPI is still compatible with Python 2.

[althonos]

Thank you! Sorry for stalling that for so long.