gclient_eval, gclient_utils: Fix Python 3.9 removal of collections members

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>

gclient_eval.py

@@ -16,13 +16,15 @@ from third_party import six
 if six.PY2:
   # We use cStringIO.StringIO because it is equivalent to Py3's io.StringIO.
   from cStringIO import StringIO
+  import collections as collections_abc
 else:
+  from collections import abc as collections_abc
   from io import StringIO
   # pylint: disable=redefined-builtin
   basestring = str
 
 
-class _NodeDict(collections.MutableMapping):
+class _NodeDict(collections_abc.MutableMapping):
   """Dict-like type that also stores information on AST nodes and tokens."""
   def __init__(self, data, tokens=None):
     self.data = collections.OrderedDict(data)
@@ -421,7 +423,7 @@ def _StandardizeDeps(deps_dict, vars_dict):
   new_deps_dict = {}
   for dep_name, dep_info in deps_dict.items():
     dep_name = dep_name.format(**vars_dict)
-    if not isinstance(dep_info, collections.Mapping):
+    if not isinstance(dep_info, collections_abc.Mapping):
       dep_info = {'url': dep_info}
     dep_info.setdefault('dep_type', 'git')
     new_deps_dict[dep_name] = dep_info
@@ -864,7 +866,7 @@ def GetRevision(gclient_dict, dep_name):
   elif isinstance(dep, basestring):
     _, _, revision = dep.partition('@')
     return revision or None
-  elif isinstance(dep, collections.Mapping) and 'url' in dep:
+  elif isinstance(dep, collections_abc.Mapping) and 'url' in dep:
     _, _, revision = dep['url'].partition('@')
     return revision or None
   else:

gclient_utils.py

@@ -28,9 +28,11 @@ import subprocess2
 
 if sys.version_info.major == 2:
   from cStringIO import StringIO
+  import collections as collections_abc
   import Queue as queue
   import urlparse
 else:
+  from collections import abc as collections_abc
   from io import StringIO
   import queue
   import urllib.parse as urlparse
@@ -1187,7 +1189,7 @@ def freeze(obj):
 
   Will raise TypeError if you pass an object which is not hashable.
   """
-  if isinstance(obj, collections.Mapping):
+  if isinstance(obj, collections_abc.Mapping):
     return FrozenDict((freeze(k), freeze(v)) for k, v in obj.items())
   elif isinstance(obj, (list, tuple)):
     return tuple(freeze(i) for i in obj)
@@ -1198,7 +1200,7 @@ def freeze(obj):
     return obj
 
 
-class FrozenDict(collections.Mapping):
+class FrozenDict(collections_abc.Mapping):
   """An immutable OrderedDict.
 
   Modified From: http://stackoverflow.com/a/2704866
@@ -1212,7 +1214,7 @@ class FrozenDict(collections.Mapping):
         operator.xor, (hash(i) for i in enumerate(self._d.items())), 0)
 
   def __eq__(self, other):
-    if not isinstance(other, collections.Mapping):
+    if not isinstance(other, collections_abc.Mapping):
       return NotImplemented
     if self is other:
       return True