Update dev requirements

Make sure mypy and type stubs are installed in dev environment. They
are already used for linting.

release_gitter_test.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import unittest
-from pathlib import Path
 from tarfile import TarFile
 from typing import Any
 from typing import Callable

requirements-dev.txt

@@ -1,4 +1,6 @@
 -e .
-pytest
 coverage
+mypy
 pre-commit
+types-requests
+types-toml