Infra: don't let generated numerical.rst trigger rebuild loop

[hugovk]

To reproduce:

$ make htmllive
...
The HTML pages are in build.
[sphinx-autobuild] Serving on http://127.0.0.1:55302
[sphinx-autobuild] Waiting to detect changes...

Then edit a file, for example peps/pep-0101.rst and it will rebuild it. So far, so good.

But then because peps/numerical.rst is regenerated, sphinx-autobuild detects a change and begins an infinite rebuild loop:

[sphinx-autobuild] Detected changes (peps/pep-0101.rst)
[sphinx-autobuild] Rebuilding...
...
The HTML pages are in build.
[sphinx-autobuild] Serving on http://127.0.0.1:55302
[sphinx-autobuild] Detected changes (peps/numerical.rst)
[sphinx-autobuild] Rebuilding...
...
The HTML pages are in build.
[sphinx-autobuild] Serving on http://127.0.0.1:55302
[sphinx-autobuild] Detected changes (peps/numerical.rst)
[sphinx-autobuild] Rebuilding...

Instead, ignore numerical.rst, just like the other generated files such as pep-0000.rst:

❯ make htmllive
...
The HTML pages are in build.
[sphinx-autobuild] Serving on http://127.0.0.1:55302
[sphinx-autobuild] Waiting to detect changes...
[sphinx-autobuild] Detected changes (peps/pep-0101.rst)
[sphinx-autobuild] Rebuilding...
...
The HTML pages are in build.
[sphinx-autobuild] Serving on http://127.0.0.1:55302

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