sslutil: another 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 "/tmp/hgtests.bc0_uk2d/install/lib/python/mercurial/sslutil.py", line 577, in wrapserversocket
+ sslcontext.minimum_version = ssl.TLSVersion.TLSv1_1
+ AttributeError: module 'ssl' has no attribute 'TLSVersion'
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true