Bump Python 3.10+ dependencies to resolve known vulnerabilities (v3.3.4)

…ndg-httpsclient

These dependencies were only used for Python 2 SNI support (gated behind
`sys.version_info < (3,)` in requests.py) and are unnecessary on Python 3,
which handles SNI natively via the stdlib ssl module.

Removes:
- pyopenssl (CVE-2026-27459, fixed in 26.0.0)
- pyasn1 (CVE-2026-30922, fixed in 0.6.3)
- ndg-httpsclient (depends on both)

Remove pyopenssl (CVE-2026-27459), pyasn1 (CVE-2026-30922), and
ndg-httpsclient from install_requires. These were only needed for
Python 2 SNI support and are dead code on Python 3.

- Remove Python 2 pyopenssl injection in pusher/requests.py
- Remove 'Python :: 2' classifier from setup.py
- Update README to note Python 2 is no longer supported

Closes #252

- aiohttp 3.13.3 -> 3.13.4
- cryptography 46.0.5 -> 46.0.6 (CVE-2026-34073)
- requests 2.32.4 -> 2.33.0 (CVE-2026-25645)

…y2-deps

security: remove vulnerable Python 2 deps (pyopenssl, pyasn1, ndg-httpsclient)

chore: consolidate dependabot security updates