The hard part was making all the tests pass; there are some subtle issues here, because apparently the future import wasn't tested very thoroughly in previous Python versions.
For example, `inspect.signature()` returned type objects normally (except for forward references), but strings with the future import. We changed it to try and return type objects by calling `typing.get_type_hints()`, but fall back on returning strings if that function fails (which it may do if there are future references in the annotations that require passing in a specific namespace to resolve).
Use an unique identifier for the different grammars documented using
the Sphinx productionlist markup.
productionlist markups of the same grammar, like "expressions" or
"compound statements", use the same identifier "python-grammar".
Sphinx 3 disallows having more than one productionlist markup with
the same name. Simply remove names in this case, since names are not
shown anyway. For example, fix the Sphinx 3 warning:
Doc/reference/introduction.rst:96: duplicate token description
of *:name, other instance in reference/expressions
Sphinx 3 requires to refer to terms with the exact case.
For example, fix the Sphinx 3 warning:
Doc/library/pkgutil.rst:71: WARNING: term Loader not found in case
sensitive match.made a reference to loader instead.
