fix(python): fix deprecated collections.abc usage (#5195)

Many classes within collections were moved to collections.abc in Python
3.3 -- their usage as imported directly from collections is now
deprecated as of Python 3.7 and will be removed soon.

The fallback import added in this PR can be removed entirely when
support for versions of Python prior to 3.3 is dropped.

python/google/protobuf/internal/containers.py

@@ -41,12 +41,16 @@ are:
 
 __author__ = 'petar@google.com (Petar Petrov)'
 
-import collections
+try:
+    # This fallback applies for all versions of Python before 3.3
+    import collections.abc as collections_abc
+except ImportError:
+    import collections as collections_abc
 import sys
 
 if sys.version_info[0] < 3:
-  # We would use collections.MutableMapping all the time, but in Python 2 it
-  # doesn't define __slots__.  This causes two significant problems:
+  # We would use collections_abc.MutableMapping all the time, but in Python 2
+  # it doesn't define __slots__.  This causes two significant problems:
   #
   # 1. we can't disallow arbitrary attribute assignment, even if our derived
   #    classes *do* define __slots__.
@@ -59,7 +63,7 @@ if sys.version_info[0] < 3:
   # verbatim, except that:
   # 1. We declare __slots__.
   # 2. We don't declare this as a virtual base class.  The classes defined
-  #    in collections are the interesting base classes, not us.
+  #    in collections_abc are the interesting base classes, not us.
   #
   # Note: deriving from object is critical.  It is the only thing that makes
   # this a true type, allowing us to derive from it in C++ cleanly and making
@@ -106,7 +110,7 @@ if sys.version_info[0] < 3:
     __hash__ = None
 
     def __eq__(self, other):
-      if not isinstance(other, collections.Mapping):
+      if not isinstance(other, collections_abc.Mapping):
         return NotImplemented
       return dict(self.items()) == dict(other.items())
 
@@ -173,13 +177,13 @@ if sys.version_info[0] < 3:
         self[key] = default
       return default
 
-  collections.Mapping.register(Mapping)
-  collections.MutableMapping.register(MutableMapping)
+  collections_abc.Mapping.register(Mapping)
+  collections_abc.MutableMapping.register(MutableMapping)
 
 else:
   # In Python 3 we can just use MutableMapping directly, because it defines
   # __slots__.
-  MutableMapping = collections.MutableMapping
+  MutableMapping = collections_abc.MutableMapping
 
 
 class BaseContainer(object):
@@ -337,7 +341,7 @@ class RepeatedScalarFieldContainer(BaseContainer):
     # We are presumably comparing against some other sequence type.
     return other == self._values
 
-collections.MutableSequence.register(BaseContainer)
+collections_abc.MutableSequence.register(BaseContainer)
 
 
 class RepeatedCompositeFieldContainer(BaseContainer):