Various collections module members were removed in Python 3.9 after being deprecated and moved to collections.abc in earlier Python 3 versions.
For accessing these members alias collections_abc as:
* collections.abc on Python 3
* collections on Python 2

Traceback (most recent call last):
  File "C:\Google\depot_tools\gclient.py", line 107, in <module>
    import gclient_eval
  File "C:\Google\depot_tools\gclient_eval.py", line 11, in <module>
    import gclient_utils
  File "C:\Google\depot_tools\gclient_utils.py", line 1201, in <module>
    class FrozenDict(collections.Mapping):
AttributeError: module 'collections' has no attribute 'Mapping'

Traceback (most recent call last):
  File "C:\Google\depot_tools\gclient.py", line 107, in <module>
    import gclient_eval
  File "C:\Google\depot_tools\gclient_eval.py", line 25, in <module>
    class _NodeDict(collections.MutableMapping):
AttributeError: module 'collections' has no attribute 'MutableMapping'

Bug: 984182
Change-Id: I7a4417978b93e29397e63764e4570a598c075bc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2043879
Auto-Submit: Raul Tambre <raul@tambre.ee>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>