gh-36640: Use GNU make tokenpool protocol to manage parallelism of doctesting
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This is useful when running our doctester in parallel with other build
steps, or several doctesters in parallel, as happens for example in
`make SAGE_CHECK=yes pypi-wheels`, and more of that after #35095.
To test:
```
MAKE="make -j14" make SAGE_NUM_THREADS=100 DEBUG_JOBCLIENT=1 ptest
```
This will make the doctester attempt to use 100 workers, but it will
only get tokens for 14 workers from `make`.
`DEBUG_JOBCLIENT=1` shows what's happening.
Upstream PR:
- https://github.com/milahu/gnumake-tokenpool/pull/3 (merged)
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- Resolves #30369
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URL: https://github.com/sagemath/sage/pull/36640
Reported by: Matthias Köppe
Reviewer(s): Michael Orlitzky
