Register Refurb as a Flake8 plugin

[jdevera]

📖 What is this PR about?

Fixes #21

In #21 it was suggested Refurb could be packaged as a Flake8 extension.

This PR tries that.

FRB is chosen as error-code, similar to the existing FURB code, but Flake8 has a max prefix length of 3.

🤔 Some learnings

• Flake8 supports extensions that will be called once per file or once per line. I chose one per file here, obviously.
• Running Refurb like this is very slow. I have not checked why but calling refurb on a directory is much faster than calling it on each file of the directory.

🔕 Disabled by default

The plugin is disabled by default and thus requires running flake8 with --enable-extensions FRB

When the flake8 plugin for a particular tool is packaged separately, it's okay to enable it by default. We can interpret its installation as an explicit desire to run it as part of flake8.

However, including the plugin as part of the tool itself, like it is done in the PR, does not allow us to reach the same conclusion. Somebody might install Refurb to run it standalone and just by doing this, we should make subsequent flake8 runs slower.

Even if running Refurb were extremely fast, I think activation should still be explicit.

Alternatively, we could have a separate package to provide the plugin.

[dsully]

I was just coming here to see about adding stdin and JSON output for refurb in order to have it work with a language server like https://github.com/jose-elias-alvarez/null-ls.nvim - this change would work however, since flake8 is already something that can be run via null-ls and is a common pre-merge validation.

Thanks for this change, hoping it will be merged!

[dosisod]

@jdevera I think the reason Refurb is running so slow is because it is using Mypy's fine grained cache, which, from what I can tell, does not get stored/retrieved from disk properly. I can't find the documentation, but I remember reading that Mypy (under certain conditions) will opt to regenerate the fine grained cache as opposed to using what is on disk. Might be wrong on that.

This was one of the reasons I didn't create Refurb as a Flake8 plugin, because you don't get as much control over how your code runs.

Some ideas for speeding things up:

• Communicate to the mypy daemon in order to speed up queries. We might be able to do this for normal Refurb as well, but just for Flake8 would be a start. I don't know if this will actually speed things up or not, but it might.
• Poke around in the mypy configuration options.

It is unfortunate that Flake8 doesn't let you run your plugin once per project instead of once per file/line. I guess they want to force you to write concurrent/optimized plugins, but that sometimes isn't feasible (such as with Mypy).