Warn datetime.{utcnow,utcfromtimestamp} usages

[sobolevn]

Closes #258

Things I did:

1. The warnings itself
2. Docs
3. Tests
4. Plus, I added tmp.py to .gitignore, because it is advertised to be used in adding-new-checks

Open questions:

• I find it quite unusual to install a dev package with pip install . and not pip install -e . Is it intentional?
• Why do you use dev-requirements.txt with poetry?

We can discuss these questions in separate issues if you wish :)

Anyways, thanks a lot for the great package!

[dosisod]

Thank you for opening this PR! I'm a bit busy atm so I'm leaving a partial review, though I'll make sure to give this a full review by this time tomorrow.

[dosisod]

Also, to answer your open questions:

• pip install . is a mistake, it should be pip install -e .. It's mainly used for CI (since it doesn't need to edit anything), but it doesn't hurt to make it editable, and is confusing for users if they run make install locally.
• I don't use Poetry, but I generated the pyproject.toml file using Poetry, so it automatically pulled in my dev dependencies. Those deps aren't kept up-to date since CI uses dev-requirements.txt. That leaves me with 2 options:
  • Delete the tool.poetry.dev-dependencies block (not ideal for Poetry users)
  • Keep the dev dependencies in sync with the dev-requirements.txt (tedious)

Personally I prefer to pin my dev dependencies, but make sure they are regularly updated. I could probably do the same thing in Poetry, though I don't know how to do dev dependency pinning in Poetry (I'll look it up later).

I'm open to solutions for handling dependencies better, so if you have any ideas on that I'd like to hear it!

[dosisod]

Overall this looks really good! I left a few of minor notes, but once those are addressed I'll go ahead and merge this in!

[dosisod]

Looks good to me! Sorry about CI failing, isort should be disabled for the test data folder. I'll fix that tomorrow.