CI: Add workaround for broken py 3.5 in GitHub Actions.

[glensc]

Workaround from #72 (comment) (from actions/setup-python#866)

For #72 (comment)

@sdelafond you may squash merge this

[glensc]

Is there a point supporting 3.5 in CI?

DEPRECATION: Python 3.5 reached the end of its life on September 13th, 2020. Please upgrade your Python as Python 3.5 is no longer maintained. pip 21.0 will drop support for Python 3.5 in January 2021. pip 21.0 will remove support for this functionality.

Successfully installed pip-20.3.4

Error: You are using pip version 20.3.4, however version 24.0 is available.

• https://github.com/libfuse/python-fuse/actions/runs/9256555310/job/25462766208?pr=73

There will be likely more and more things to break adding more maintenance burden. If somebody needs fix for 3.5 they can still submit a PR, just not tested in CI anymore.

[sdelafond]

Cheers!