Deprecate backend_webagg.ServerThread.

[anntzer]

It is clearly just for internal use, can be replaced by a plain Thread(target=...), and removing it gets rid of a large block in the webagg api docs that just restates the standard Thread docstring (https://matplotlib.org/devdocs/api/backend_webagg_api.html#matplotlib.backends.backend_webagg.ServerThread).

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).

[tacaswell]

The docs failure looks unrelated?

[oscargus]

I think so as well. Isn't there a theoretical risk of that happening when building in parallel and now it happened to trigger?

[tacaswell]

Yes, but I do not have a good theory as to how a race condition could cause errors that look like:

Traceback (most recent call last):
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/cmd/build.py", line 281, in build_main
    app.build(args.force_all, args.filenames)
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/application.py", line 347, in build
    self.builder.build_update()
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 310, in build_update
    self.build(to_build,
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 326, in build
    updated_docnames = set(self.read())
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 431, in read
    self._read_parallel(docnames, nproc=self.app.parallel)
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 487, in _read_parallel
    tasks.join()
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/util/parallel.py", line 105, in join
    if not self._join_one():
  File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/util/parallel.py", line 126, in _join_one
    raise SphinxParallelError(*result)
sphinx.errors.SphinxParallelError: ValueError: gallery/pyplots/dollar_ticks is already noted as redirecting to gallery/ticks/dollar_ticks

Sphinx parallel build error:
ValueError: gallery/pyplots/dollar_ticks is already noted as redirecting to gallery/ticks/dollar_ticks

[tacaswell]

I restarted the job

✔ 11:32:43 $ ack "redirect-from" | grep ticks
examples/ticks/date_index_formatter.py:6:.. redirect-from:: /gallery/text_labels_and_annotations/date_index_formatter
examples/ticks/date_index_formatter.py:7:.. redirect-from:: /gallery/ticks/date_index_formatter2
examples/ticks/dollar_ticks.py:8:.. redirect-from:: /gallery/pyplots/dollar_ticks
examples/ticks/fig_axes_customize_simple.py:8:.. redirect-from:: /gallery/pyplots/fig_axes_customize_simple

says there is nothing wrong so maybe two workers got examples/ticks/dollar_ticks.py and then it collided with itself? I also do not see any other failures on cricle so lets hope the restart "fixes" this and worry about tracking down transient errors when they become more common.

[timhoffm]

I vaguely remember to have seen that Sphinx error before. Anyway, it's not related to this code and we can deal with it if it hits again.