gmp install with mamba on macOS doesn't work

[aaron-skydio]

From #369 (comment)

Along these same lines, the README.md should also be modified to say that gmp must be installed with brew and cannot be installed with mamba on macOS. When I install gmp with mamba, I get the following error:

      CMake Error at /private/var/folders/0j/gcrqfhps14b11tyxczhdzdv80000gn/T/pip-build-env-qp81m144/normal/lib/python3.11/site-packages/cmake/data/share/cmake-3.26/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
        Could NOT find GMP (missing: GMP_LIBRARIES GMP_INCLUDE_DIRS)
      Call Stack (most recent call first):
        /private/var/folders/0j/gcrqfhps14b11tyxczhdzdv80000gn/T/pip-build-env-qp81m144/normal/lib/python3.11/site-packages/cmake/data/share/cmake-3.26/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
        cmake/FindGMP.cmake:19 (find_package_handle_standard_args)
        CMakeLists.txt:198 (find_package)