Basic Constraints path_length can be None

[Richard Vernon]

"env/lib/python3.6/site-packages/cursive/certificate_utils.py", line 344, in verify
    if constraints.path_length < chain_length:
TypeError: '<' not supported between instances of 'NoneType' and 'int'

From https://cryptography.io/en/latest/x509/reference/?highlight=path_length#cryptography.x509.BasicConstraints.path_length

> This attribute only has meaning if ca is true. If ca is true then a path length of None means there’s no restriction on the number of subordinate CAs in the certificate chain.

The comparison needs a None check as well.