Replace vendored dkim with dkimpy package

- Remove emails/packages/dkim/ (vendored fork of dkimpy 0.5.3)
- Use upstream dkimpy package (1.1.8) from PyPI
- Add dkimpy to install_requires in setup.py
- Update imports in signers.py, exc.py, test_dkim.py
- Pass PEM bytes instead of pre-parsed key dict to dkim.sign()
- Fix test dnsfunc lambda to accept timeout kwarg
- Remove mypy exclusion for emails/packages/

Closes #196

Author: lavr

emails/exc.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from .packages.dkim import DKIMException
+from dkim import DKIMException
 
 
 class HTTPLoaderError(Exception):