[3.6] bpo-29887: test_normalization handles PermissionError (#1196)

[vstinner]

Skip test_normalization.test_main() if download raises a permission
error.
(cherry picked from commit d13d547)

[vstinner]

@Mariatta: Oh, your backport broke "x86 Gentoo Installed with X 3.6" buildbot, you forgot to backport my following fix. Usually, I prefer to only modify the master branch to not break too many buildbots, and it's not uncommon that I need a second change to fix all issues ;-)

So here is my backport of the second fix. You need to include this commit in your 3.5 backport as well: PR #2272.

[vstinner]

The buildbot failure:

======================================================================
FAIL: test_main (test.test_normalization.NormalizationTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/3.6.ware-gentoo-x86.installed/build/target/lib/python3.6/test/test_normalization.py", line 42, in test_main
    check=check_version)
PermissionError: [Errno 13] Permission denied: '/buildbot/buildarea/3.6.ware-gentoo-x86.installed/build/target/lib/python3.6/test/data/NormalizationTest.txt'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/buildbot/buildarea/3.6.ware-gentoo-x86.installed/build/target/lib/python3.6/test/test_normalization.py", line 44, in test_main
    self.fail(f"Could not retrieve {TESTDATAURL}")
AssertionError: Could not retrieve http://www.pythontest.net/unicode/9.0.0/NormalizationTest.txt

http://buildbot.python.org/all/builders/x86%20Gentoo%20Installed%20with%20X%203.6/builds/420/steps/test/logs/stdio

Don't worry about the bug. I reviewed your PR and forgot that I made a second change, it's my fault.