Allow newer cryptography and add Python 3.14 tox

[achamayou]

Summary

This PR updates the project metadata and test fixtures for the current supported dependency/runtime landscape:

• widens the direct cryptography dependency constraint from <47 to <50
• adds Python 3.14 to the tox environment list and tox-gh-actions mapping as py314
• refreshes the bundled server certificate fixture used by certificate validation sample tests

Why the certificate fixture changed

The existing tests/keys/certs/server.pem fixture expired on 2026-03-16. As of the current date, the certificate validation sample tests fail because cryptography.x509.verification correctly rejects the expired certificate at validation time.

The refresh keeps the existing server key and CA material, but regenerates the server certificate with a new validity window and updates the embedded x5c value in the JWK fixtures. That preserves the intent of the tests while making them pass under real certificate validation rules again.

Cryptography dependency note

This PR allows recent cryptography releases at the python-cwt direct dependency level by changing the range to <50. This is a metadata compatibility step toward accepting the current 49.x release line.

Poetry still cannot resolve cryptography==49.0.0 today because the required pyhpke dependency currently declares cryptography >=42.0.1,<47. A follow-up PR is needed in pyhpke to allow newer cryptography versions there too; once that is released, python-cwt should be able to pick up the newer cryptography line through the full dependency graph.

Validation

• poetry run tox -l lists py314
• poetry run pytest -ra tests passes: 1345 passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.01%. Comparing base (421cee2) to head (a444481).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #670   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files          32       32           
  Lines        3445     3445           
=======================================
  Hits         3342     3342           
  Misses        103      103           

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[achamayou]

@dajiaji this is a minimal set of changes to enable usage of python-cwt with newer versions of the cryptography library and newer versions of the Python interpreter.