bpo-34011: Fixes missing venv files and other tests

[zooba]

Mostly related to test fixes for when Python is running outside of a regular build directory. The only product impact is for bpo-34011

https://bugs.python.org/issue34011

[vsajip]

The change in venv/__init__.py seems OK to me. I thought the venv tests worked when I ran them with an installed Python (you might remember some discussion back in July on Zulip about trying to build a WiX installer for Python - that was to test this issue using an installed Python). Not sure what I missed there.

[zooba]

@vsajip If you put Python on PATH when you installed, then it would work. But if you don't select that option, it can't find python38.dll automatically.

[zooba]

And I don't follow Zulip, so you can't rely on me noticing any conversations on there at all (ah okay, I was actually involved in this one :) )

[vsajip]

@vsajip If you put Python on PATH when you installed, then it would work. But if you don't select that option, it can't find python38.dll automatically.

Ah, right, that could well explain it.

[pablogsal]

This buildbot is failing when raising when creating subprocess.CalledProcessError:

https://buildbot.python.org/all/#/builders/40/builds/1135/steps/3/logs/stdio

test_defaults (test.test_venv.BasicTest) ... ok
test_executable (test.test_venv.BasicTest) ... ok
test_executable_symlinks (test.test_venv.BasicTest) ... skipped 'Needs symlinks'
test_isolation (test.test_venv.BasicTest) ... ok
test_overwrite_existing (test.test_venv.BasicTest) ... ok
test_prefixes (test.test_venv.BasicTest) ... ok
test_prompt (test.test_venv.BasicTest) ... ok
test_symlinking (test.test_venv.BasicTest) ... skipped 'Needs symlinks'
test_unicode_in_batch_file (test.test_venv.BasicTest) ... ERROR
test_unoverwritable_fails (test.test_venv.BasicTest) ... ok
test_upgrade (test.test_venv.BasicTest) ... ok
test_devnull (test.test_venv.EnsurePipTest) ... ok
test_explicit_no_pip (test.test_venv.EnsurePipTest) ... ok
test_no_pip_by_default (test.test_venv.EnsurePipTest) ... ok
test_with_pip (test.test_venv.EnsurePipTest) ... ok
======================================================================
ERROR: test_unicode_in_batch_file (test.test_venv.BasicTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\test_venv.py", line 302, in test_unicode_in_batch_file
    out, err = check_output(
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\test_venv.py", line 37, in check_output
    raise subprocess.CalledProcessError(
TypeError: __init__() takes from 3 to 5 positional arguments but 6 were given
----------------------------------------------------------------------

I will prepare a PR fixing this

[pablogsal]

More Windows buildbots are failing:

https://buildbot.python.org/all/#/builders/32/builds/1706
https://buildbot.python.org/all/#/builders/3/builds/1633

[pablogsal]

@zooba @vsajip There may be more problems with this Pull Request than just the problem above, can you take a look at the buildbots?