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.
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors