Import Mapping and Sequence from compat in python_api::approx

Related to #3339

Fixes a DeprecationWarning on Python 3.7

Adds Mapping to compat

Author: hroncok

_pytest/compat.py

@@ -40,11 +40,11 @@
 
 if _PY3:
     from collections.abc import MutableMapping as MappingMixin  # noqa
-    from collections.abc import Sequence  # noqa
+    from collections.abc import Mapping, Sequence  # noqa
 else:
     # those raise DeprecationWarnings in Python >=3.7
     from collections import MutableMapping as MappingMixin  # noqa
-    from collections import Sequence  # noqa
+    from collections import Mapping, Sequence  # noqa
 
 
 def _format_args(func):

_pytest/python_api.py

@@ -426,7 +426,7 @@ def approx(expected, rel=None, abs=None, nan_ok=False):
        __ https://docs.python.org/3/reference/datamodel.html#object.__ge__
     """
 
-    from collections import Mapping, Sequence
+    from _pytest.compat import Mapping, Sequence
     from _pytest.compat import STRING_TYPES as String
     from decimal import Decimal