Dependencies/backends: Require cryptography

[houzefa-abba]

Remove dependencies on ecdsa, pyasn1, rsa.
cryptography is now a required dependency.
Remove non-cryptography backends.

In the process, also fix a non-initialized instance of CryptographyEc.SECP256 in test_incorrect_public_key_hmac_signing.

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This is an opinionated PR, hence version bump to 4.0.0.

I know python-jose was working fine with optional backends, but doing this simplifies packaging / security analysis by a lot.
Non-cryptography backend deps are obsolete anyway:

• ecdsa: CVE-2024-23342 Timing Attack tlsfuzzer/python-ecdsa#330 (comment)

  I don't want people to use this library in production environments...
  It's a teaching tool, it's a testing tool, it's absolutely not an production grade implementation.

• rsa: Python-RSA has been archived sybrenstuvel/python-rsa#245
  Archived in April 2025

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#400 is a similar PR but it only removes ecdsa; mine goes further.