Add basic mypy linting

Based on pytest setup, though more lax. Currently only the tests are
annotated so only helpful for them.

Fix #721.

setup.cfg

@@ -58,3 +58,19 @@ setproctitle = setproctitle
 
 [flake8]
 max-line-length = 100
+
+[mypy]
+mypy_path = src
+# TODO: Enable this & fix errors.
+# check_untyped_defs = True
+disallow_any_generics = True
+ignore_missing_imports = True
+no_implicit_optional = True
+show_error_codes = True
+strict_equality = True
+warn_redundant_casts = True
+warn_return_any = True
+warn_unreachable = True
+warn_unused_configs = True
+# TODO: Enable this & fix errors.
+# no_implicit_reexport = True