sslutil: properly detect which TLS versions are supported by the ssl module
For the record, I contacted the CPython developers to remark that
unconditionally defining ssl.PROTOCOL_TLSv1_1 / ssl.PROTOCOL_TLSv1_2 is
problematic:
https://github.com/python/cpython/commit/
6e8cda91d92da72800d891b2fc2073ecbc134d98#r
39569316
sslutil: remove dead code (that failed if only TLS 1.0 is available)
We ensure in setup.py that TLS 1.1 or TLS 1.2 is present.