bpo-34179: Make sure decimal context doesn't affect other tests

[bbayles]

The use of decimal.getcontext in test_time.py affects other modules that use the decimal module.

The PR changes the tests to use decimal.localcontext, which doesn't have this problem.

After

$ ./python -m test test_time test_statistics
Run tests sequentially
0:00:00 load avg: 2.53 [1/2] test_time
0:00:02 load avg: 2.53 [2/2] test_statistics

== Tests result: SUCCESS ==

All 2 tests OK.

Total duration: 3 sec 80 ms
Tests result: SUCCESS

Before

$ ./python -m test test_time test_statistics
Run tests sequentially
0:00:00 load avg: 2.54 [1/2] test_time
0:00:02 load avg: 2.66 [2/2] test_statistics
test test_statistics failed -- multiple errors occurred; run in verbose mode for details
test_statistics failed

== Tests result: FAILURE ==

1 test OK.

1 test failed:
    test_statistics

Total duration: 3 sec 87 ms
Tests result: FAILURE

https://bugs.python.org/issue34179

[skrah]

Thanks for the patch. In this case I would not make a NEWS entry, since the change does not affect users.

[bbayles]

I've reverted the NEWS entry - I never know whether to add one. bedevere-bot immediately gives PRs a ❌ if there's not one, so I usually go ahead and do.

[skrah]

Yes, the bots aren't very flexible and NEWS entries somewhat subject to personal taste. I usually add one if I'd really want to be informed as a user, other people practically always add one.

[bedevere-bot]

@skrah: Please replace # with GH- in the commit message next time. Thanks!

[miss-islington]

Thanks @bbayles for the PR, and @skrah for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6, 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-8383 is a backport of this pull request to the 3.7 branch.

[bedevere-bot]

GH-8384 is a backport of this pull request to the 3.6 branch.