CLN: Remove "flake8: noqa" from files

Just some minor house-cleaning to cut down on the number of search
results found <a href="https://github.com/pandas-
dev/pandas/search?utf8=%E2%9C%93&q=flake8%3A+noqa&type=">here</a>.

Author: gfyoung <[email protected]>

Closes #15842 from gfyoung/flake8-noqa-clean and squashes the following commits:

5d1edeb [gfyoung] CLN: Make test_compat.py flake8-able
f9079ff [gfyoung] CLN: Make exceptions.py flake8-able
0e236f5 [gfyoung] CLN: Make test_format.py flake8-able

Author: gfyoung

pandas/tests/computation/test_compat.py

@@ -1,8 +1,4 @@
-
-# flake8: noqa
-
 import pytest
-from itertools import product
 from distutils.version import LooseVersion
 
 import pandas as pd
@@ -32,7 +28,7 @@ def test_compat():
 @pytest.mark.parametrize('parser', expr._parsers)
 def test_invalid_numexpr_version(engine, parser):
     def testit():
-        a, b = 1, 2
+        a, b = 1, 2  # noqa
         res = pd.eval('a + b', engine=engine, parser=parser)
         tm.assert_equal(res, 3)