Skip mast remote tests that need boto3 if it is not installed

[eerovaher]

If astroquery is installed with pip install astroquery[test] then pytest --remote-data=any -k test_get_cloud_uri astroquery/mast results in two tests failing with ModuleNotFoundError: No module named 'boto3' because although boto3 is listed among the all dependencies, it is not listed among the test dependencies:

astroquery/setup.cfg

Lines 136 to 140 in aa9ce56

	all=
	mocpy>=0.9
	astropy-healpix
	boto3
	regions

astroquery/setup.cfg

Lines 127 to 131 in aa9ce56

	test=
	pytest-astropy
	matplotlib
	pytest-dependency
	pytest-rerunfailures

The tests therefore cannot assume that boto3 is installed. There are two simple solutions to this problem. One would be to add boto3 to the test dependencies, but adding a whole new dependency only for the two remote tests in mast is in my opinion excessive. This pull request implements the other simple option, which is to check whether boto3 is installed in the two tests that require it and to skip them if it isn't.

[codecov]

Codecov Report

Merging #2755 (890f617) into main (aa9ce56) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2755   +/-   ##
=======================================
  Coverage   66.04%   66.04%           
=======================================
  Files         233      233           
  Lines       17927    17927           
=======================================
  Hits        11840    11840           
  Misses       6087     6087           

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[bsipocz]

I'm OK with this, though don't think adding boto3 to test dependencies would be such an issue, after all we pull in mpl for the sake of a few doctests.