setup: require that Python has TLS 1.1 or TLS 1.2


setup: require that Python has TLS 1.1 or TLS 1.2

This ensures that Mercurial never downgrades the minimum TLS version from
TLS 1.1+ to TLS 1.0+ and enables us to remove that compatibility code.

It is reasonable to expect that distributions having Python 2.7.9+ or having
backported modern features to the ssl module (which we require) have a OpenSSL
version supporting TLS 1.1 or TLS 1.2, as this is the main reason why
distributions would want to backport these features.

TLS 1.1 and TLS 1.2 are often either both enabled or both not enabled.
However, both can be disabled independently, at least on current Python /
OpenSSL versions.

For the record, I contacted the CPython developers to remark that
unconditionally defining ssl.PROTOCOL_TLSv1_1 / ssl.PROTOCOL_TLSv1_2 is