• Simplify the implementation of repeat_delay: instead of switching
  callbacks back and forth, just set the timer interval at each
  iteration to either the normal delay or the end-of-loop delay.

• Fix support for repeat_delay when frames is an iterable: the
  previous implementation handled looping in iter_frames and therefore
  _step never saw the end of the iterable. Instead, just iterate
  through frames once in iter_frames, but still tee the iterable
  so that _step can take care of repeating the animation.

Can be checked with

import matplotlib.pyplot as plt
import matplotlib.animation as animation

fig, ax = plt.subplots()
line, = ax.plot(range(10))

def animate(i):
    line.set_ydata(range(i, i + 10))  # update the data.
    return line,

ani = animation.FuncAnimation(
    fig, animate, frames=range(5), interval=1000, repeat_delay=2000, blit=True, save_count=50)
plt.show()

Previously repeat_delay would not be applied (this was likely intro'ed by
#14068, so this PR could be considered as a bugfix, hence the milestoning).

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• 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).