Swap deprecated np.int to int

[robertdstein]

Since numpy v1.20, the use of aliases such as np.int have been deprecated (see e.g https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations). The recommended solution is to instead useint, which is equivalent. This PR replaces for four instances of np.int in sfdmap.py with int.

When I run the unit tests, I would previously get 38 warnings with numpy v1.22, but now get none.

[temuller]

@kbarbary I would really appreciate if this pull request is accepted!

[temuller]

@kbarbary could you please check this PR?

[mlincett]

Note that a fork is being maintained at https://github.com/AmpelAstro/sfdmap2

I have mentioned this in #2.