Eliminate warnings from test suite

[eslavich]

This fixes or hides the following warnings that come up when running our tests:

PytestDeprecationWarning: direct construction of AsdfSchemaFile has been deprecated, please use AsdfSchemaFile.from_parent
PytestDeprecationWarning: direct construction of AsdfSchemaItem has been deprecated, please use AsdfSchemaItem.from_parent
PytestDeprecationWarning: direct construction of AsdfSchemaExampleItem has been deprecated, please use AsdfSchemaExampleItem.from_parent

The pytest plugin has been updated to use from_parent when available.

AsdfConversionWarning: tag:stsci.edu:asdf/wcs/celestial_frame-1.1.0 is not recognized, converting to raw Python data structure
AsdfConversionWarning: tag:stsci.edu:asdf/wcs/icrs_coord-1.1.0 is not recognized, converting to raw Python data structure

Some of the tests open a file that references schemas that have been deprecated. At some point we'll want to create new test data, but it seems like we should wait until the dust settles on moving schemas around. For the time being I just asked pytest to ignore the warning.

YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.

Changed asdf.tests.helpers to use safe_load instead.

The PR also adds a not-required Travis build that converts warnings into errors.

Resolves #789

[codecov]

Codecov Report

Merging #799 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #799   +/-   ##
=======================================
  Coverage   94.07%   94.07%           
=======================================
  Files          42       42           
  Lines        5013     5013           
=======================================
  Hits         4716     4716           
  Misses        297      297           

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[jdavies-st]

Looks good. Just a question below.

[jdavies-st]

Can the -W ignore::asdf.exceptions.AsdfDeprecationWarning:asdf.asdftypes be removed here as we capture and ignore it elsewhere?

[jdavies-st]

Or does this override the filterwarnings directive in [tool:pytest]?

[eslavich]

Right, that's the issue -- pytest uses the last matched rule to determine if the warning should be ignored, and filterwarnings seems to get loaded first.

[astrofrog]

Thanks for the fix! Would it be possible to release a new version with this fix? 🙏