diff --git a/Lib/collections/abc.py b/Lib/collections/abc.py
--- a/Lib/collections/abc.py
+++ b/Lib/collections/abc.py
@@ -7,6 +7,7 @@
 """
 
 from abc import ABCMeta, abstractmethod
+import os
 import sys
 
 __all__ = ["Hashable", "Iterable", "Iterator",
@@ -588,6 +589,7 @@
         return default
 
 MutableMapping.register(dict)
+MutableMapping.register(os._Environ)
 
 
 ### SEQUENCES ###
diff --git a/Lib/os.py b/Lib/os.py
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -631,9 +631,30 @@
 
 
 # Change environ to automatically call putenv(), unsetenv if they exist.
-from collections.abc import MutableMapping
 
-class _Environ(MutableMapping):
+class _EnvironMeta(type):
+    # one-shot wonder: replaces itself with type when resolving baseclass
+    # See issue #19218.
+    def __new__(meta, name, bases, ns):
+        bases = (type('_FakeMutableMapping', (object,), {'__slots__': ()}),)
+        return type.__new__(meta, name, bases, ns)
+    def _resolve_lazy_baseclass(cls):
+        from collections.abc import MutableMapping
+        cls.__class__ = type('_EnvironMetaReplaced', (type,), {})
+        cls.__bases__ = (MutableMapping,)
+        del cls.__getattr__
+        del cls.get
+    def __getattribute__(cls, name):
+        if name in ('__class__', '__bases__'):
+            cls._resolve_lazy_baseclass()
+            return getattr(cls, name)
+        return type.__getattribute__(cls, name)
+    def __getattr__(cls, name):
+        cls._resolve_lazy_baseclass()
+        return getattr(cls, name)
+
+
+class _Environ(metaclass=_EnvironMeta):
     def __init__(self, data, encodekey, decodekey, encodevalue, decodevalue, putenv, unsetenv):
         self.encodekey = encodekey
         self.decodekey = decodekey
@@ -686,6 +707,20 @@
             self[key] = value
         return self[key]
 
+    # For issue #19218 (removed when baseclass is resolved).
+    def __getattr__(self, name):
+        self.__class__._resolve_lazy_baseclass()
+        return getattr(self, name)
+
+    # For issue #19218 (removed when baseclass is resolved).
+    # Implemented for the sake of the site module.
+    def get(self, name, default=None):
+        try:
+            return self[name]
+        except KeyError:
+            return default
+
+
 try:
     _putenv = putenv
 except NameError: