Make tox actions work on Debian 10

[ilmari]

• Debian 10 ships Python 3.7, so make that basepython for all envs
• Tox 3.7.0 doesn't seem to like trailing comments on dep lines

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

[clokep]

@ilmari I don't see anything wrong with this PR, but I'm not 100% sure what it is fixing. Is tox unable to run on Debian 10 without this change?

[ilmari]

@clokep Yes, without this, running tox -e check_codestyle,check-newsfragment fails, because /usr/bin/python3 is Python 3.7.

I just realised a better option might be to make them reference {[base]basepython} so it only has to be changed in one place and is kept in sync across the different commands, or to leave out the basepython completely, so that it'll accept whatever version of Python 3 is in $PATH.

ilmari@ilmari-thinkpad:~/src/synapse (develop $)$ tox -e check_codestyle,check-newsfragment
check_codestyle recreate: /home/ilmari/src/synapse/.tox/check_codestyle
ERROR: InterpreterNotFound: python3.6
check-newsfragment create: /home/ilmari/src/synapse/.tox/check-newsfragment
ERROR: InterpreterNotFound: python3.6
_______________________________________________________ summary _______________________________________________________
ERROR:  check_codestyle: InterpreterNotFound: python3.6
ERROR:  check-newsfragment: InterpreterNotFound: python3.6

[erikjohnston]

I think the latter makes sense to me, my worry here is that this will break on systems that don't have a python3.7? I haven't had a chance to really think about how this all works, but if we can make it not depend on a particular python then I think that would save me from having to think about this (I'm assuming that different versions of python won't result in different results for these linters).

[erikjohnston]

Thanks!