Drop compatibility with Python 3.5

[gilles-peskine-arm]

Move to Python 3.6 as the minimum Python version that actually works. This is the oldest Python version that we promise to support in Mbed TLS 2.28 LTS.

Take advantage of this to upgrade pylint (and also mypy and cryptography) to the last version that still supports Python 3.6.

Resolves https://github.com/Mbed-TLS/mbedtls/issues/9092. Not sufficient for https://github.com/Mbed-TLS/mbedtls/issues/9091, unfortunately: this version of Pylint still crashes on some files with Python ≥3.11.

Prerequisites:

• [x] Necessary: Get rid of Python 3.5 on our CI.
• [ ] Conflicts with https://github.com/Mbed-TLS/mbedtls/pull/9025 and that one is more complicated and more urgent so I want to get it in first.

PR checklist

• [x] changelog no (we're still within the same documented platform requirements)
• [ ] 3.6 backport TODO
• [ ] 2.28 backport TODO
• tests CI, but also need some manual checks for scripts that aren't supported on the CI: TODO