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'

mercurial/sslutil.py

@@ -312,7 +312,7 b' def wrapsocket(sock, keyfile, certfile, '
     # is loaded and contains that removed CA, you've just undone the user's
     # choice.
 
-    if util.safehasattr(ssl, 'PROTOCOL_TLS_CLIENT'):
+    if util.safehasattr(ssl, 'TLSVersion'):
         # python 3.7+
         sslcontext = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
         minimumprotocol = settings[b'minimumprotocol']