Support Python 3.9.5 and beyond

[al-the-x]

Currently the highest version of Python supported is 3.9.4, as defined in the Python helper python_versions.rb and limited by the version of pyenv installed into the Python executor Dockerfile.

My immediate need is for Dependabot to use Python 3.9.5, as it currently chokes in our work repo. (We can downgrade back to 3.9.4 temporarily, of course, but we're trying to stay up-to-date.) Please point me in the right direction to submit a PR for this.

Longer term, how could Dependabot support updating it's own language version dependencies when new releases are available, to minimize the manual work required to update it?

Related:

• https://github.com/dependabot/dependabot-core/blob/main/python/lib/dependabot/python/python_versions.rb#L3-L30
• build(deps): bump pipenv from 2018.11.26 to 2021.5.29 in /python/helpers #3817
• https://www.python.org/dev/peps/pep-0596/#bugfix-releases
• https://www.python.org/dev/peps/pep-0619/

Python 3.9.5 was released 2021-05-03, Ave 3.9.6 is expected 2021-06-28 according to the PEPs above. Version 3.10.0 is expected in 2021-10 and beta 1 was released with 3.5.9 on 05-03