sslutil: remove code checking for presence of ssl.SSLContext
Now that we require the presence of ssl.SSLContext in setup.py, we can remove
this code.
setup: require a Python version with modern SSL features
This increases the minimum security baseline of Mercurial and enables us to
remove compatibility code for supporting older, less secure Python versions.
sslutil: set `_canloaddefaultcerts` to `True` if `ssl.SSLContext` is present
The `load_default_certs()` method was already present when `ssl.SSLContext`
was backported to Python 2.7 (https://hg.python.org/cpython/rev/
221a1f9155e2).