bpo-42208: Pass tstate to _PyGC_CollectNoFail()

[vstinner]

Move private _PyGC_CollectNoFail() to the internal C API.

Remove the private _PyGC_CollectIfEnabled() which was just an alias
to the public PyGC_Collect() function since Python 3.8.

Rename functions:

• collect() => gc_collect_main()
• collect_with_callback() => gc_collect_with_callback()
• collect_generations() => gc_collect_generations()

https://bugs.python.org/issue42208

[pablogsal]

This PR broke test_gc:

test test_gdb failed
Verify "py-up" followed by "py-down" ... ok
======================================================================
FAIL: test_gc (test.test_gdb.PyBtTests)
Verify that "py-bt" indicates if a thread is garbage-collecting
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_gdb.py", line 861, in test_gc
    self.assertIn('Garbage-collecting', gdb_output)
AssertionError: 'Garbage-collecting' not found in 'Breakpoint 1 at 0x102f5e04: file Python/bltinmodule.c, line 1173.\n[Thread debugging using libthread_db enabled]\nUsing host libthread_db library "/lib64/libthread_db.so.1".\n\nBreakpoint 1, builtin_id (self=, v=42) at Python/bltinmodule.c:1173\n1173\t{\nBreakpoint 2: file Modules/gcmodule.c, line 420.\n\nBreakpoint 2, update_refs (containers=) at Modules/gcmodule.c:421\n421\t    PyGC_Head *gc = GC_NEXT(containers);\nTraceback (most recent call first):\n  <built-in method collect of module object at remote 0x7fffea9f7d70>\n  File "<string>", line 4, in foo\n  File "<string>", line 6, in bar\n  File "<string>", line 7, in <module>\n'
----------------------------------------------------------------------
Ran 46 tests in 64.862s
FAILED (failures=1)
1 test failed again:
    test_gdb
== Tests result: FAILURE then FAILURE ==
410 tests OK.
10 

Example breakage:

https://buildbot.python.org/all/#/builders/90/builds/246/steps/5/logs/stdio

[vstinner]

This PR broke test_gc:

Oops, I wrote PR #23041 to update python-gdb.py.

It's strange: test_gdb passed on Travis CI!? test_gdb was skipped on the Ubuntu job since gdb is not installed there:
python/core-workflow#383