bpo-39573: Use Py_TYPE() macro in object.c

[vstinner]

Replace direct acccess to PyVarObject.ob_size with usage of the
Py_SIZE() macro.

https://bugs.python.org/issue39573

[codecov]

Codecov Report

Merging #18398 into master will increase coverage by 1.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #18398       +/-   ##
===========================================
+ Coverage   82.11%   83.19%    +1.08%     
===========================================
  Files        1955     1570      -385     
  Lines      588906   414397   -174509     
  Branches    44429    44429               
===========================================
- Hits       483572   344754   -138818     
+ Misses      95679    59996    -35683     
+ Partials     9655     9647        -8     

Impacted Files	Coverage Δ
Lib/distutils/tests/test_bdist_rpm.py	30.00% <0.00%> (-65.00%)	⬇️
Lib/distutils/command/bdist_rpm.py	7.63% <0.00%> (-56.88%)	⬇️
Lib/test/test_urllib2net.py	76.92% <0.00%> (-13.85%)	⬇️
Lib/test/test_smtpnet.py	78.57% <0.00%> (-7.15%)	⬇️
Lib/ftplib.py	63.85% <0.00%> (-6.06%)	⬇️
Lib/test/test_ftplib.py	87.11% <0.00%> (-4.72%)	⬇️
Tools/scripts/db2pickle.py	17.82% <0.00%> (-3.97%)	⬇️
Tools/scripts/pickle2db.py	16.98% <0.00%> (-3.78%)	⬇️
Lib/test/test_socket.py	71.94% <0.00%> (-3.77%)	⬇️
Lib/test/test_asyncio/test_base_events.py	91.84% <0.00%> (-3.30%)	⬇️
... and 435 more

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