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).
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h