There's a merged pull request on the void-packages repository

python3-mpmath: depend on python3-gmpy2
https://github.com/void-linux/void-packages/pull/33511

Description:
By default mpmath uses python's builtin long integers which are very
slow. Installing python3-gmpy2 makes it use gmp, mpfr, mpc for that,
which is asymptotically faster.

After #33481 it was agreed to add it to depends.

Also:
 - remove INSTALL.msg; for plotting, mpmath itself gives a warning.
 - Adopt.

[ci-skip] since this will fail before #33481 is merged, but nothing is really changed