Add doctest_fail_fast option to exit after the first failed test.

[tillahoffmann]

This PR adds a doctest_fail_fast option for the doctest extension. When the flag is set, the doctest builder exits on the first failure it encounters. This behavior mirrors pytest -x to exit on the first failed test.

[tillahoffmann]

Thanks for the review, @AA-Turner!

It looks like there are now a few failing tests for 3.13 because the pathlib module is organized differently. Do you know how I might be able to fix that?

➜  ~ docker run --rm python:3.14-rc python -c 'import pathlib; print(pathlib.Path)'
<class 'pathlib.Path'>
➜  ~ docker run --rm python:3.13 python -c 'import pathlib; print(pathlib.Path)'
<class 'pathlib._local.Path'>

[AA-Turner]

It looks like there are now a few failing tests for 3.13 because the pathlib module is organized differently. Do you know how I might be able to fix that?

Fixed in e01e42f, thanks for alerting me to the problem.

A