Misc fixes

[skirpichev]

• consistently use "floating-point"
• correct return type for mpfr.__floor__, __ceil__ and __trunc__, closes Expected behavior of math.ceil, math.floor, math.trunc, and round #143
• correct mpq.__float__(), closes Invalid mpq.__float__? #498
• mark fallthroughs, closes Intentional fallthroughs? #501

I'll mark also additional issues (merging this will close them too):

• closes Impending release of gmpy2 2.2.0a2 #462
• closes gmpy-2.1.5 fails to compile with python-3.12 #446
• closes New version? 🤔 #419
• closes test failures with mpfr 4.2.1 (change in MPFR formatted functions) #418
• closes travis.yml : make latexpdf error on osx #168
• closes Setup (build) fails with Mac Osx 10.12 and Python3.6 #163
• closes convert to and from binary #116
• closes No wheel available for Windows and Python 3.5 #111
• closes Search broken on docs #73
• closes patch: mpz_bitcount, faster mpmath_normalize #21
• closes Pip install build fails, can't find mpc.h #451

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.60%. Comparing base (224745b) to head (44e879d).
Report is 37 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
+ Coverage   85.52%   85.60%   +0.07%     
==========================================
  Files          50       50              
  Lines       11656    11650       -6     
  Branches     2202     2201       -1     
==========================================
+ Hits         9969     9973       +4     
+ Misses       1687     1677      -10     

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[casevh]

Thanks for all the clean-up.