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 unix-permissions no-root
$ hg init a
$ cd a
$ echo foo > b
$ hg add b
$ hg ci -m "b"
$ chmod -w .hg/store
$ cd ..
$ hg clone a b
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 97310831fa1a
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ chmod +w a/.hg/store # let test clean up
$ cd b
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
$ cd ..