Refactoring to drop deprecated usages in pyOpenSSL & drop Python 3.7 support

[adferrand]

Fixes #181

Caution: This PR introduces backward incompatible changes to the library. A major version of josepy should be issued when this PR is merged.
Caution: Defensive PR on Certbot should be merged first -> certbot/certbot#9993

This PR removes the APIs deprecated by pyopenssl in version 24.2+.

As a consequence, several changes of josepy public API are made:

• josepy.ComparableX509 is removed since the underlying wrapped object is now objects from cryptography API (Certificate or CertificateSigningRequest) and these objects support equality checks.
• methods encode_csr, decode_csr, encode_cert and decode_cert now manipulates objects from cryptography API
• underlying tests are updated.

Aside these changes, the support of Python 3.7 is dropped, given this PR will require a new major version anyway.

[zoracon]

This should resolve advisories: (needs Python 3.7 dropped)

• https://github.com/certbot/josepy/security/dependabot/26
• https://github.com/certbot/josepy/security/dependabot/26
• https://github.com/certbot/josepy/security/dependabot/25

[jharrisonSV]

Any movement on this? 👀

[bmw]

I opened #186 to drop Python 3.7 and fix security alerts.

As for this PR in itself, we will likely do this and/or deprecate this library entirely as discussed at certbot/certbot#8322 and the comments on certbot/certbot#9993 in the next few months. We just need to map out what that transition will look like for the code in https://github.com/certbot/certbot.