remotenames: work around move of ABCs in collections
This starts warning in Python 3.7, and will break in 3.8. The import
is performed in a curious place because `import collections.abc`
explodes badly on Python 2.7, and `from collections import abc` flunks
our import checker.
Differential Revision: https://phab.mercurial-scm.org/D2847
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors