Fix spacing in arxiv format (and the build workflow)

[misharash]

These changes should fix #34:

• replace the tilde ~ with space only if not preceded by the backslash \ using a regular expression;
• replace the LaTeX space preceded by the backslash with a usual space.

[misharash]

The build checks seem to have failed due to a deprecated Ubuntu version, tried to fix that in a new commit (and also increased the Python 3 version).

[misharash]

Apparently Python 2.7 is no longer supported in GitHub Actions either. Tried to fix the build by removing it and adding a few more Python 3 versions.

[kadrlica]

Starting at Python 3.8 is probably fine. Thanks for these changes btw!

[misharash]

Sorry, I am rather new to the workflows. I enabled them in my fork, and will try to make sure it works there first.

[misharash]

Finally, the package is built successfully in my fork. (Still true with the next three commits.)

[kadrlica]

Sorry that this took so long. I also didn't realize that I'd need to approve/launch each workflow for a contributor who was outside the hosting organization. Thanks for all the work debugging the workflow.

I've approved your PR, so you should be able to go ahead an merge (otherwise I can do it).

[misharash]

GitHub says I need write access to this repository to merge PRs, which I don't have.

It also appeared to me that the added replacement of \ might have an issue similar to the (resolved) one with tilde: \\ -> \ . On the other hand, it is not clear to me what the correct behavior should be in this case. Moreover, it does not seem very relevant to the names of authors.