test_support metadata: Mark data type as "optional"

```
test_support/test_support/metadata.py:274: error: Argument 10 to "StandardMetadata" has incompatible type "list[tuple[str, str]]"; expected "list[tuple[str, str | None]]"  [arg-type]
test_support/test_support/metadata.py:274: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
test_support/test_support/metadata.py:274: note: Consider using "Sequence" instead, which is covariant
test_support/test_support/metadata.py:275: error: Argument 11 to "StandardMetadata" has incompatible type "list[tuple[str, str]]"; expected "list[tuple[str, str | None]]"  [arg-type]
test_support/test_support/metadata.py:275: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
test_support/test_support/metadata.py:275: note: Consider using "Sequence" instead, which is covariant
```

Not sure if `str | None` is valid in all Python versions we support, but
I think `Optional[str]` and `Union[str, None]` if I'm not mistaken, more
or less say the same thing.  So let's go with the most concise and
widely supported of these.

test_support/test_support/metadata.py

@@ -171,7 +171,7 @@ def is_at_least(required: str, *, v=metadata_version):
 
     authors = _parse_contributors("Author", get("Author", []), "Author-email", get("Author-email", []))
 
-    maintainers: List[Tuple[str, str]]
+    maintainers: List[Tuple[str, Optional[str]]]
     if is_at_least("1.2"):
         maintainers = _parse_contributors(
             "Maintainer", get("Maintainer", []), "Maintainer-email", get("Maintainer-email", [])