Refactor astroquery/utils/system_tools.py and its tests

[eerovaher]

See the commit messages for details.

[codecov]

Codecov Report

Merging #2306 (2920b77) into main (f99e1ba) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2306   +/-   ##
=======================================
  Coverage   62.84%   62.84%           
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  Files         130      130           
  Lines       16841    16837    -4     
=======================================
- Hits        10584    10582    -2     
+ Misses       6257     6255    -2     

Impacted Files	Coverage Δ
astroquery/utils/system_tools.py	83.33% <100.00%> (+8.33%)	⬆️

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[bsipocz]

I recall some issues around non-availablity of gzip on windows, thus this whole workaround was added in #1613? It was done after we dropped python2 support though

@pllim - do you see any issue with this PR?

[pllim]

I can see gzip on Windows with Python 3.8. If it passes in your Windows CI, probably ok?

[bsipocz]

OK, thanks for confirming. Let's try to see this then, thanks @eerovaher for the cleanup.

[eerovaher]

I'll try to clear any confusion there may be about the availability of gzip on different platforms. There exists a program called gzip which has nothing to do with Python, and which is not guaranteed to be installed by all users. Before #1613 astroquery tried to use that program, but its availability had to be checked. What #1613 did was it replaced the external gzip with the Python standard library module also called gzip, which can be safely assumed to be available for all astroquery users without any need to check its presence.

[bsipocz]

Yes, I suppose part of the issues came with the fact that the standalone, UNIX gzip could deal with the fits.Z files, but it broke when we switched. Or something else, I can't recall and some of that may be lost in discussions on slack, but it certainly raised as an issue during the precious PR and thus we ended up with the extra checks.