Rename to flake8 async

[jakkdl]

First steps in renaming the project:

1. Update documentation to refer to flake8-async instead of flake8-trio
2. Add name history to readme, expand it a bit in general.
3. rename the executable

In the second commit I rename all the error codes. This was largely done with a search&replace of TRIO->ASYNC across all files, and then cleaning up the mess.

• several places assumed the length of the error code to be 7, which is now incorrect. (This is to turn e.g. TRIO103_anyio/TRIO103_trio into the actual error code. The _anyio/_trio is to generate different suggestion depending on which library you're using.)
  • The number 8 is now saved in a constant, or we use re.match, to avoid the magic number.

Third commit renames Flake8TrioVisitor to Flake8AsyncVisitor

Review the commits separately to maybe retain some sanity. I think the tests should be thorough enough to pick up anything problematic though, so don't think we have to review this super thoroughly.

Remaining steps:

• rename the repository to python-trio/flake8-async
  • update URL's. (I updated some to point to python-trio/flake8-trio instead of ZacHD/flake8-trio)
• Rename files/directories. I'll do this in a separate commit, maybe even a separate PR, once the content of files has been updated, since github fails to diff the content of a file if it's renamed in the same commit
  • Rename the eval & autofix files, removing the temporary renaming functions.
  • rename the flake8_trio directory, the flake8_trio/visitors/flake8triovisitor.py file, and the tests/test_flake8_trio.py file.