math/py-gmpy2-devel: fix build with PYTHON_DEFAULT <3.7 >3.9

• Explicitly specify correct sphinx-build binary to use. Use versioned binary, as unversioned one may not be available (such as when building with default python=3.10; in this case PY_FLAVOR is py38 as this port does not yet support python 3.10, so it pulls sphinx-build@py38 which does not install sphinx-build as 3.8 is not default python version).
• Fix LICENSE and add LICENSE_FILE
• Do not try to install docs in build phase

Approved by: portmgr blanket