Remove ineffective exclusion of Arcs without parent Axes.

[anntzer]

The if not hasattr(self, 'axes'): raise RuntimeError(...) check was
ineffectual, as artists now always have an Axes attribute, which can
just be None for some artists. In fact, small Arcs are drawn just fine
without a parent Axes; e.g.

from pylab import *
from matplotlib.patches import *
fig = figure()
fig.add_artist(Ellipse((.2, .2), .1, .3, angle=45))  # for comparison
fig.add_artist(Arc((.2, .2), .1, .3, angle=45, theta1=0, theta2=45))

works just fine. Remove the check, and adjust the docs accordingly.

On the other hand, large arcs did previously fail,
but that occurred a bit further down, when computing
transforms.BboxTransformTo(self.axes.bbox) (self.axes is None -->
AttributeError). Fix that by using the figure bbox in that case (as the
point is to limit the drawing to the unclipped area, which is the whole
figure for Arcs without a parent Axes).

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]

I am surprised this has no tests.

[tacaswell]

I am pretty sure that the coverage error here is a false-negative and I believe this will be further tested by #23340 .

[tacaswell]

Test failures appear to all be the font issues.