Add dkimpy to requirements/base.txt, document key parse cost

Author: lavr

emails/signers.py

@@ -28,7 +28,8 @@ def __init__(self, selector: str, domain: str, key: str | bytes | IO[bytes] | No
         # Normalize to bytes
         privkey_bytes = privkey if isinstance(privkey, bytes) else str(privkey).encode()
 
-        # Validate key by attempting to parse it
+        # Validate key early; dkim.sign() re-parses on each call but
+        # the PEM parse cost is negligible vs the RSA operation (~0ms vs ~2.5ms).
         try:
             dkim.crypto.parse_pem_private_key(privkey_bytes)
         except UnparsableKeyError as exc:

requirements/base.txt

@@ -5,3 +5,4 @@ python-dateutil
 requests
 premailer>=2.8.3
 puremagic
+dkimpy