Small fixes and a bug?

[ekaterinailin]

Hi @bmorris3,

first of all, thanks for this nicely documented lean piece of programming! fleck turns out to work great with flares, too! While trying out the software, I stumbled over a few small issues, mostly cosmetic, which I suggest incorporating in a future of release of fleck:

First, I suggest, bumping up fleck to at least Python 3.8.

Second, I suggest following adjustments to core.py:

• np.int >>> np.int32 to remove a deprecation warning from numpy
• self.phases[:, np.newaxis, np.newaxis] >>> self.phases[..., np.newaxis, np.newaxis] so that fleck works with higher dimensional input (e.g., if accommodating dynamic flaring regions instead of static spots)
• convert inc_stellar from rad to deg in generate_spots to make it more consistent with input variables that are given in deg, e.g. min_latitude and max_latitude.
• removing inplace multiplication in generate_spots because that, for some reason, does not work with astropy units (this is something a have seen failing in one of the recent travis runs, so I don't think that's a local issue)

Cheers, Ekaterina