TST: Pin bleach until they fix deprecation warning

[pllim]

Description

Merging this would hopefully fix CI for unrelated PRs.

This pull request is to address this deprecation warning caused by bleach 3.2.0 release last night.

.../astropy/utils/xml/writer.py:195: in <lambda>
    self.xml_escape_cdata = lambda x: bleach.clean(x, **clean_kwargs)
.../bleach/__init__.py:84: in clean
    return cleaner.clean(text)
.../bleach/sanitizer.py:175: in clean
    filtered = BleachSanitizerFilter(
.../bleach/sanitizer.py:273: in __init__
    return super(BleachSanitizerFilter, self).__init__(source, **kwargs)
...
E       DeprecationWarning: html5lib's sanitizer is deprecated;
see https://github.com/html5lib/html5lib-python/issues/443 and
please let us know if Bleach is unsuitable for your needs

I debated with myself whether we should pin or ignore warning. I think pinning is more explicit as a reminder that this is temporary and should be undone in near future. We tend to forget why we ignore the warnings and leave them there.

p.s. If this blocks RC testing, you might want to backport or something. Not sure.

Also see mozilla/bleach#557 and html5lib/html5lib-python#443 (comment) .

[astrofrog]

We could also just pin it in ci-helpers? (in the pin file?)

[astrojuanlu]

bleach 3.2.1 was released with this fix 🎉

[pllim]

@astrojuanlu , that's great news! If you can confirm that CI fixed itself, you can go ahead and close this without merge. 😄

[astrojuanlu]

The RTD PR builder here turned green #10727 so I think this can be closed!