hgweb: avoid using `sslutil.modernssl`

`sslutil.modernssl` is going to be removed. Since the point of using this
attribute was to check the importability of the `sslutil`, a different
attribute can be used. `sslutil.wrapserversocket` is used because it’s anyway
used a few lines below.

mercurial/hgweb/server.py

@@ -313,7 +313,7 b' class _httprequesthandlerssl(_httpreques'
         try:
             from .. import sslutil
 
-            sslutil.modernssl
+            sslutil.wrapserversocket
         except ImportError:
             raise error.Abort(_(b"SSL support is unavailable"))