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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