bpo-37473: Don't import importlib ASAP in tests

[vstinner]

bpo-15386, bpo-37473: test_import, regrtest and libregrtest no longer
import importlib as soon as possible, as the first import, "to test
bpo-15386".

Sort test_import imports.

https://bugs.python.org/issue37473

[vstinner]

@brettcannon, @ncoghlan: Are you ok to remove these hacks?

It seems like https://bugs.python.org/issue15386 is already tested by test_import.test_there_can_be_only_one(). "import imp" was replaced with "import _imp" incommit be7e49f:

commit be7e49fd820318509cd8b4dbde479c552f74ef62
Author: Nick Coghlan <ncoghlan@gmail.com>
Date:   Fri Jul 20 23:40:09 2012 +1000

    Close #15386: There was a loophole that meant importlib.machinery and imp would sometimes reference an uninitialised copy of importlib._bootstrap

At least, I can say that importlib breaks quickly with the following change:

diff --git a/Lib/importlib/__init__.py b/Lib/importlib/__init__.py
index 0c73c505f9..130bc19537 100644
--- a/Lib/importlib/__init__.py
+++ b/Lib/importlib/__init__.py
@@ -9,7 +9,7 @@ __all__ = ['__import__', 'import_module', 'invalidate_caches', 'reload']
 # modules would get an uninitialised copy of the source version, instead
 # of a fully initialised version (either the frozen one or the one
 # initialised below if the frozen one is not available).
-import _imp  # Just the builtin component, NOT the full Python module
+import imp  # Just the builtin component, NOT the full Python module
 import sys
 
 try:

[brettcannon]

As long as there's another test then I'm fine with the change.

[aeros]

After reading through the issues and doing some testing on the latest version, I checked to see if the issue is still occurring:

I haven't worked out how yet, but importlib.machinery is managing to bypass the replacement of importlib._bootstrap with _frozen_importlib:

Since FileFinder was moved from _bootstrap to _bootstrap_external I replaced the reference when I ran through the process in the interpreter:

Based upon the outputs, it looks like the issue was solved at some point by having importlib._bootstrap_external.FileFinder and importlib.machinery.FileFinder refer to the same class, _frozen_importlib_external.FileFinder. To ensure this is happening, an assertion such as this could be used:

# ...
self.assertEqual(importlib.machinery.FileFinder, 
importlib._bootstrap_external.FileFinder, 
msg = "Both locations should refer to the same class.")

It might be a bit bandage solution still since it does not explain why the override was occurring in the first place, but at least it no longer happens with FileFinder.

[vstinner]

I merged your PR. If someone considers than test_there_can_be_only_one() is not enough, please add a new test.

[ncoghlan]

importlib initialisation changed quite a bit when the external import bootstrapping was split out to a separate step, so it's quite plausible that the previous odd behaviour was a result of FileFinder being defined in the core import system bootstrap module.