Fix Python 3.7 collections ABC classes deprecation warning

Fixes #55

expects/matchers/built_in/contain.py

@@ -1,7 +1,11 @@
 # -*- coding: utf-8 -*
 
 import functools
-import collections
+
+try:
+    import collections.abc as collections_abc
+except ImportError:
+    import collections as collections_abc
 
 from .. import Matcher, default_matcher
 from ...texts import plain_enumerate
@@ -10,9 +14,9 @@
 
 class contain(Matcher):
     _NON_NORMALIZED_SEQUENCE_TYPES = (
-        collections.Iterator,
-        collections.MappingView,
-        collections.Set
+        collections_abc.Iterator,
+        collections_abc.MappingView,
+        collections_abc.Set
     )
 
     def __init__(self, *expected):
@@ -35,7 +39,7 @@ def _match(self, subject):
         return self._matches(subject)
 
     def _is_not_a_sequence(self, value):
-        return not isinstance(value, collections.Sequence)
+        return not isinstance(value, collections_abc.Sequence)
 
     def _matches(self, subject):
         reasons = []

expects/matchers/built_in/have_keys.py

@@ -1,6 +1,9 @@
 # -*- coding: utf-8 -*
 
-import collections
+try:
+    import collections.abc as collections_abc
+except ImportError:
+    import collections as collections_abc
 
 from .. import Matcher, default_matcher
 from ...texts import plain_enumerate
@@ -14,7 +17,7 @@ def _match(self, subject):
         return self._matches(subject)
 
     def _not_a_dict(self, value):
-        return not isinstance(value, collections.Mapping)
+        return not isinstance(value, collections_abc.Mapping)
 
     def _matches(self, subject):
         args, kwargs = self._expected

expects/matchers/built_in/start_end_with.py

@@ -2,6 +2,11 @@
 
 import collections
 
+try:
+    import collections.abc as collections_abc
+except ImportError:
+    import collections as collections_abc
+
 from .. import Matcher
 from ...texts import plain_enumerate
 from ... import _compat
@@ -19,7 +24,7 @@ def _match(self, subject):
 
     def _is_unordered_dict(self, subject):
         return (
-            isinstance(subject, collections.Mapping) and
+            isinstance(subject, collections_abc.Mapping) and
             not isinstance(subject, collections.OrderedDict)
         )