Revert "CI: Address linting errors in flake8 >= 3.8.1 (pandas-dev#34152)"

This reverts commit f196e8d.

Author: simonjayhawkins

pandas/core/arrays/interval.py

@@ -1070,7 +1070,6 @@ def mid(self):
         points) and is either monotonic increasing or monotonic decreasing,
         else False.
         """
-
     # https://github.com/python/mypy/issues/1362
     # Mypy does not support decorated properties
     @property  # type: ignore

pandas/core/internals/concat.py

@@ -381,7 +381,7 @@ def is_uniform_join_units(join_units):
     """
     return (
         # all blocks need to have the same type
-        all(isinstance(ju.block, type(join_units[0].block)) for ju in join_units)
+        all(type(ju.block) is type(join_units[0].block) for ju in join_units)
         and  # noqa
         # no blocks that would get missing values (can lead to type upcasts)
         # unless we're an extension dtype.

pandas/tests/groupby/test_categorical.py

@@ -497,9 +497,9 @@ def test_dataframe_categorical_ordered_observed_sort(ordered, observed, sort):
         aggr[aggr.isna()] = "missing"
     if not all(label == aggr):
         msg = (
-            "Labels and aggregation results not consistently sorted\n"
-            f"for (ordered={ordered}, observed={observed}, sort={sort})\n"
-            f"Result:\n{result}"
+            f"Labels and aggregation results not consistently sorted\n"
+            + "for (ordered={ordered}, observed={observed}, sort={sort})\n"
+            + "Result:\n{result}"
         )
         assert False, msg
 

setup.cfg

@@ -19,7 +19,6 @@ ignore =
     W504,  # line break after binary operator
     E402,  # module level import not at top of file
     E731,  # do not assign a lambda expression, use a def
-    E741,  # ambiguous variable name 'l' (GH#34150)
     C406,  # Unnecessary list literal - rewrite as a dict literal.
     C408,  # Unnecessary dict call - rewrite as a literal.
     C409,  # Unnecessary list passed to tuple() - rewrite as a tuple literal.