sslutil: use proper attribute to select python 3.7+
The previous attribute was python 3.6+, but guarded a python 3.7+ block.
Using the correct attribute avoids: File "/usr/lib64/python3.6/site-packages/mercurial/sslutil.py", line 334, in wrapsocket sslcontext.minimum_version = ssl.TLSVersion.TLSv1_1 AttributeError: module 'ssl' has no attribute 'TLSVersion'