Fix #5777. Don't warn when applying default style

[mdboom]

No description provided.

[jenshnielsen]

The context manager is badly named it does not actually catch any warnings it just resets the warning filter when leaving the context. I think you have to do:

with warnings.catch_warnings(record=True):
    warnings.simplefilter("ignore", UserWarning)
    _apply_style(rcParamsDefault)

[jenshnielsen]

If I understand this correctly the with warnings.catch_warnings(record=True) means that the warning is only showed once but it is still showed exactly once

[mdboom]

If I understand this correctly the with warnings.catch_warnings(record=True) means that the warning is only showed once but it is still showed exactly once

For me, at least, the warnings aren't shown at all, due to the record=True. But on further consideration, a think a new kwarg is better -- it will be more efficient, as there's no need to generate the warning object in the first place, only to catch and suppress it elsewhere. Updating this PR...

[jenshnielsen]

It's shown once in https://travis-ci.org/matplotlib/matplotlib/jobs/99718127 but that might be do to other reasons.

[mdboom]

Let's see if the latest commit here fixes that warning in the doc build. If not, there's probably something else to track down.

[jenshnielsen]

There is still a single /home/travis/build/matplotlib/matplotlib/lib/matplotlib/style/core.py:53: UserWarning: Style includes a parameter, 'backend', that is not related to style. Ignoring so that must presumably be triggered by some other code path.