Add invoke as task runner, ruff and fix mypy errors

scripts/smtp-convert-secret-key-to-password.py

@@ -39,11 +39,11 @@ TERMINAL = "aws4_request"
 VERSION = 0x04
 
 
-def sign(key, msg):
+def sign(key: bytes, msg: str) -> bytes:
     return hmac.new(key, msg.encode("utf-8"), hashlib.sha256).digest()
 
 
-def calculate_key(secret_access_key):
+def calculate_key(secret_access_key: str) -> str:
     signature = sign(("AWS4" + secret_access_key).encode("utf-8"), DATE)
     signature = sign(signature, REGION)
     signature = sign(signature, SERVICE)
@@ -54,7 +54,7 @@ def calculate_key(secret_access_key):
     return smtp_password.decode("utf-8")
 
 
-def main():
+def main() -> None:
     if sys.version_info[0] < 3:
         raise Exception("Must be using Python 3")