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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).