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'

mercurial/sslutil.py

@@ -546,7 +546,7 b' def wrapserversocket('
                 _(b'referenced certificate file (%s) does not exist') % f
             )
 
-    if util.safehasattr(ssl, 'PROTOCOL_TLS_SERVER'):
+    if util.safehasattr(ssl, 'TLSVersion'):
         # python 3.7+
         sslcontext = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
         sslcontext.options |= getattr(ssl, 'OP_NO_COMPRESSION', 0)