pycompat: add util.urlerr util.urlreq classes for py3 compat
python3 url.request and url.error are mapped as util.urlreq/util.urlerr
python2 equivalents from urllib/urllib2 are mapped according to the py3
hierarchy
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])