Add type annotations (#934)

Refs e.g. python/typeshed#9455

Co-authored-by: Spencer Brown <spencerb21@live.com>
Co-authored-by: Aarni Koskela <akx@iki.fi>

Author: 3 people

.coveragerc

@@ -3,3 +3,4 @@ exclude_lines =
     NotImplemented
     pragma: no cover
     warnings.warn
+    if TYPE_CHECKING: