Move compatibility imports to compat.py and fix linting

_pytest/assertion/util.py

@@ -5,11 +5,7 @@ import pprint
 import _pytest._code
 import py
 import six
-import sys
+from ..compat import Sequence
-if sys.version_info >= (3, 4):
-    from collections.abc import Sequence
-else:
-    from collections import Sequence
 
 u = six.text_type
 

_pytest/compat.py

@@ -38,6 +38,14 @@ PY35 = sys.version_info[:2] >= (3, 5)
 PY36 = sys.version_info[:2] >= (3, 6)
 MODULE_NOT_FOUND_ERROR = 'ModuleNotFoundError' if PY36 else 'ImportError'
 
+if _PY2:
+    # those raise DeprecationWarnings in Python >=3.7
+    from collections.abc import MutableMapping as MappingMixin  # noqa
+    from collections.abc import Sequence  # noqa
+else:
+    from collections import MutableMapping as MappingMixin  # noqa
+    from collections import Sequence  # noqa
+
 
 def _format_args(func):
     return str(signature(func))

_pytest/mark/structures.py

@@ -1,18 +1,13 @@
-import sys
-if sys.version_info >= (3, 4):
-    from collections.abc import MutableMapping as MappingMixin
-else:
-    from collections import MutableMapping as MappingMixin
-from collections import namedtuple
-import warnings
-from operator import attrgetter
 import inspect
+import warnings
+from collections import namedtuple
+from operator import attrgetter
 
 import attr
-from ..deprecated import MARK_PARAMETERSET_UNPACKING
-from ..compat import NOTSET, getfslineno
 from six.moves import map
 
+from ..compat import NOTSET, getfslineno, MappingMixin
+from ..deprecated import MARK_PARAMETERSET_UNPACKING
 
 EMPTY_PARAMETERSET_OPTION = "empty_parameter_set_mark"