math/py-contourpy: unbreak build with non-default python (+)

Countourpy fails to find versioned pybind11-config which leads to configure error

pybind11-config found: NO
Did not find CMake 'cmake'
Found CMake: NO
Run-time dependency pybind11 found: NO (tried pkgconfig and config-tool)

../meson.build:23:15: ERROR: Dependency "pybind11" not found, tried pkgconfig and config-tool

Fix it by providing BINARY_ALIAS

Reported by: poudriere failure
Approved by: portmgr blanket