Set proper User-agent header (Python-webchecker/<version>).
Set proper User-agent header (Python-webchecker/<version>).

When -x is combined with -q, still do the checking, but don't print
the error in this phase -- they are reported by report_errors().

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator