Let approx() work on more generic sequences

approx() was updated in 9f3122fe to work better with numpy arrays,
however at the same time the requirements were tightened from
requiring an Iterable to requiring a Sequence - the former being
tested only on interface, while the latter requires subclassing or
registration with the abc.

Since the ApproxSequence only used __iter__ and __len__ this commit
reduces the requirement to only what's used, and allows unregistered
Sequence-like containers to be used.

Since numpy arrays qualify for the new criteria, reorder the checks so
that generic sequences are checked for after numpy arrays.

src/_pytest/python_api.py

@@ -11,8 +11,9 @@ from six.moves import zip
 
 import _pytest._code
 from _pytest.compat import isclass
+from _pytest.compat import Iterable
 from _pytest.compat import Mapping
-from _pytest.compat import Sequence
+from _pytest.compat import Sized
 from _pytest.compat import STRING_TYPES
 from _pytest.outcomes import fail
 
@@ -182,7 +183,7 @@ class ApproxMapping(ApproxBase):
                 raise _non_numeric_type_error(self.expected, at="key={!r}".format(key))
 
 
-class ApproxSequence(ApproxBase):
+class ApproxSequencelike(ApproxBase):
     """
     Perform approximate comparisons where the expected value is a sequence of
     numbers.
@@ -518,10 +519,14 @@ def approx(expected, rel=None, abs=None, nan_ok=False):
         cls = ApproxScalar
     elif isinstance(expected, Mapping):
         cls = ApproxMapping
-    elif isinstance(expected, Sequence) and not isinstance(expected, STRING_TYPES):
-        cls = ApproxSequence
     elif _is_numpy_array(expected):
         cls = ApproxNumpy
+    elif (
+        isinstance(expected, Iterable)
+        and isinstance(expected, Sized)
+        and not isinstance(expected, STRING_TYPES)
+    ):
+        cls = ApproxSequencelike
     else:
         raise _non_numeric_type_error(expected, at=None)