Log Message:
py-rsa: updated to 4.0

Version 4.0:
- Removed deprecated modules:
    - rsa.varblock
    - rsa.bigfile
    - rsa._version133
    - rsa._version200
- Removed CLI commands that use the VARBLOCK/bigfile format.
- Ensured that PublicKey.save_pkcs1() and PrivateKey.save_pkcs1() always return \ 
bytes.
- Dropped support for Python 2.6 and 3.3.
- Dropped support for Psyco.
- Miller-Rabin iterations determined by bitsize of key.
- Added function rsa.find_signature_hash() to return the name of the hashing
  algorithm used to sign a message. rsa.verify() now also returns that name,
  instead of always returning True.
- Add support for SHA-224 for PKCS1 signatures.
- Transitioned from requirements.txt to Pipenv for package management.