Use python.dependency() instead of INCLUDEPY to fix mesonpy buid in PEP517

[woodtp]

The PyORBIT3 build can fail while using a PEP 517-compliant build front end (e.g., pixi) if the temporary build environment is placed within the source tree.

pixi add --pypi 'pyorbit @ file:///path/to/PyORBIT3'

results in

../src/meson.build:252:6: ERROR: Tried to form an absolute path to a dir in the source tree.
      You should not do that but use relative paths instead, for
      directories that are part of your project.

This PR makes a small modification to meson.build declaring Python as an explicit dependency so that the appropriate paths to Python headers/libs are resolved automatically by meson.