tools/urls-check: some cleanups
tools/urls-check: some cleanups

Move user agent to the top as a constant.

Move the PatternFly exception also to the top by converting the
exceptions list into fnmatch patterns.

Get rid of the non-idiomatic use of /usr/bin/env to find python3.

Clean up some string formatting to use f-strings and be more generally
readable.

mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit