Add test to ensure Numba imports without warnings.

With view of catching issues like that fixed in numba#6486 as the
entire code base is not yet run under flake8.

Also fixes this test module and makes it flake8 compliant.

.flake8

@@ -171,7 +171,6 @@ exclude =
     numba/tests/test_extended_arg.py
     numba/tests/test_alignment.py
     numba/tests/test_multi3.py
-    numba/tests/test_import.py
     numba/tests/test_overlap.py
     numba/tests/test_array_attr.py
     numba/tests/test_array_methods.py

numba/tests/test_import.py

@@ -1,31 +1,42 @@
+import unittest
 import subprocess
 import sys
 
 from numba.tests.support import TestCase
-import unittest
 
 
 class TestNumbaImport(TestCase):
     """
     Test behaviour of importing Numba.
     """
 
+    def run_in_subproc(self, code, flags=None):
+        if flags is None:
+            flags = []
+        cmd = [sys.executable,] + flags + ["-c", code]
+        popen = subprocess.Popen(cmd, stdout=subprocess.PIPE,
+                                 stderr=subprocess.PIPE)
+        out, err = popen.communicate()
+        if popen.returncode != 0:
+            msg = "process failed with code %s: stderr follows\n%s\n"
+            raise AssertionError(msg % (popen.returncode, err.decode()))
+        return out, err
+
     def test_laziness(self):
         """
         Importing top-level numba features should not import too many modules.
         """
         # A heuristic set of modules that shouldn't be imported immediately
-        blacklist = [
-            'cffi',
-            'distutils',
-            'numba.cuda',
-            'numba.cpython.mathimpl',
-            'numba.cpython.randomimpl',
-            'numba.tests',
-            'numba.core.typing.collections',
-            'numba.core.typing.listdecl',
-            'numba.core.typing.npdatetime',
-            ]
+        blacklist = ['cffi',
+                     'distutils',
+                     'numba.cuda',
+                     'numba.cpython.mathimpl',
+                     'numba.cpython.randomimpl',
+                     'numba.tests',
+                     'numba.core.typing.collections',
+                     'numba.core.typing.listdecl',
+                     'numba.core.typing.npdatetime',
+                     ]
         # Sanity check the modules still exist...
         for mod in blacklist:
             if mod not in ('cffi',):
@@ -38,13 +49,18 @@ def test_laziness(self):
             print(list(sys.modules))
             """
 
-        popen = subprocess.Popen([sys.executable, "-c", code],
-                                 stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-        out, err = popen.communicate()
-        if popen.returncode != 0:
-            raise AssertionError("process failed with code %s: stderr follows\n%s\n"
-                                 % (popen.returncode, err.decode()))
-
+        out, _ = self.run_in_subproc(code)
         modlist = set(eval(out.strip()))
         unexpected = set(blacklist) & set(modlist)
         self.assertFalse(unexpected, "some modules unexpectedly imported")
+
+    def test_no_accidental_warnings(self):
+        # checks that importing Numba isn't accidentally triggering warnings due
+        # to e.g. deprecated use of import locations from Python's stdlib
+        code = "import numba"
+        flags = ["-Werror",]
+        self.run_in_subproc(code, flags)
+
+
+if __name__ == '__main__':
+    unittest.main()