Hard-wired Python dependencies

Since Python3.10, the contents of the file src/einsteinpy.egg-info/requires.txt entail the creation of an unwanted dependency on python3-numba. As python3-einsteinpy can work without python3-numba, it is better to make it a recommendation.

This change (hard-wiring the dependencies rather than let ${python3:Depends} do the job) allows the build process to work more smoothly, when python3-numba is not yet ready for unstable.