Fix *most of* the Deprecation Warnings in surfarray_test

[MyreMylar]

Noticed that there were several fixable Deprecation Warnings coming out of our surfarray tests.

I feel in general that we should try and minimise the outputting of 'expected' warnings from our tests so that we are more likely to find the unexpected ones when running a full test run.

I spotted the warnings by running:

pytest test/surfarray_test.py -s

There are two main fixes applied:

1. Using..

with self.assertWarns(DeprecationWarning):

as a context manager when we are deliberately calling a deprecated function at the moment - we can remove tests like this if/when the deprecated function itself is finally removed.

2. Applying the recommendation from Numpy's deprecation warning where we have deprecated behaviour:

DeprecationWarning: NumPy will stop allowing conversion of out-of-bound Python integers to integer arrays.  The conversion of -10185016 to uint32 will fail in the future.
  For the old behavior, usually:
      np.array(value).astype(dtype)
  will give the desired result (the cast overflows).
    arr[:5, 6:] = surf.map_rgb(palette[3])

[damusss]

Nice fix, thanks :3

[ankith26]

LGTM, thanks! 🎉