[Github Actions] Apply lang/python label to files outside of src/python (#32788)

Follow-up to https://github.com/grpc/grpc/pull/32751. I finally broke
down and put together a hacky way to mess with this manually. [This
hacked up version of the
implementation](https://gist.github.com/gnossen/f2f334b448eab73f383d65d5206056b6)
adds some unit tests to make sure it does exactly what we want.

The final issue was that [this
block](49b9dfa644/.github/labeler.yml (L39))
would never trigger unless _all_ of the explicitly mentioned files was
added there. The fix is to pull them each individually into their own
[`MatcherConfig`](d8f73a2893/src/labeler.ts (L6))
object with a single [`any`
field](d8f73a2893/src/labeler.ts (L8)).

.github/labeler.yml

@@ -36,7 +36,6 @@ lang/php:
 - src/php/**
 
 lang/python:
-- any:
 - bazel/python_rules.bzl
 - examples/python/**
 - requirements.bazel.txt