Do not mangle Python stack traces in sphinx-build

[saraedum]

This upstreams part of Debian's u2-better-sphinx-failure-modes.patch by Ximin Luo: we should not .strip() the lines that come out of Sphinx because this breaks indent in stack traces. Instead we should only rstrip() to remove the trailing newline.

After this one line change, I also wanted to make this doctestable, so I had to remove the global variable (which at least to me feels like a hack anyway.)

Finally, I added a copyright header sourced from the git log.

CC: @infinity0

Component: doctest framework

Author: Julian Rüth

Branch: 67aff77

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/25160

[saraedum]

Branch: u/saraedum/sphinxbuild-stack

[saraedum]

Commit: 18afd6d

[saraedum]

New commits:

9cc176a	Do not break indent on Python stack traces
795cecb	Make sphinxbuild.py doctestable
18afd6d	Add COPYRIGHT header

[sagetrac-git]

Changed commit from 18afd6d to 0936a5d

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

4e714a2	Add checks for binder
84d541c	fix README
73b963b	POSIX compliant test, [ instead of [[.
0d5dd3d	Fix sage-jupyter command in entrypoint
7cb69d7	Mention that timings may vary
6a02f35	Wrap BaseException so Pool.get() aborts
6a7640d	Add doctest for #25161
fcbfe1c	More standard logging usage
fc5bf18	Merge remote-tracking branch 'trac/u/saraedum/docbuild-base-exception' into u/saraedum/gitlabci
0936a5d	Merge remote-tracking branch 'trac/u/saraedum/sphinxbuild-stack' into u/saraedum/gitlabci

[sagetrac-git]

Changed commit from 0936a5d to 0477b3c

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

3f0c2f0	Add doctest for #25160
0477b3c	Check for errors before ignoring output

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

ca66c03

Revert "Check for errors before ignoring output"

[sagetrac-git]

Changed commit from 0477b3c to ca66c03

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

5881359

Remove raise exception logic