Run lint _once_, not once per Python version

AFAICS, there is no difference in behavior when running `mypy` or
`ruff check .` in a Python environment based on v3.7 or one based on
v3.12. These tools should behave the same irrespective of the Python
version in the surrounding environment. Hence, there is no longer any
need to run nox's "lint" session under each of our supported Python
versions.

This will divide the time spent on linters by 6.

noxfile.py

@@ -109,7 +109,7 @@ def self_test(session):
     session.run("fawltydeps")
 
 
-@nox.session(python=python_versions)
+@nox.session
 def lint(session):
     install_groups(session, include=["lint"])
     session.run("mypy")