bpo-39943: Clean up marshal.c.

[serhiy-storchaka]

• Add consts.
• Remove redundant casts and checks.
• Use concrete C API macros.

Supersedes #19210.

https://bugs.python.org/issue39943

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu Shared 3.x has failed when building commit 2c003ef.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/101/builds/624) and take a look at the build logs.
4. Check if the failure is related to this commit (2c003ef) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/101/builds/624

Failed tests:

• test_builtin

Failed subtests:

• test_input_no_stdout_fileno - test.test_builtin.PtyTests

Summary of the results of the build (if available):

== Tests result: FAILURE then FAILURE ==

402 tests OK.

10 slowest tests:

• test_multiprocessing_spawn: 4 min 49 sec
• test_concurrent_futures: 4 min 5 sec
• test_unparse: 3 min 23 sec
• test_tokenize: 3 min 10 sec
• test_capi: 2 min 53 sec
• test_gdb: 2 min 43 sec
• test_lib2to3: 2 min 27 sec
• test_multiprocessing_forkserver: 2 min 9 sec
• test_asyncio: 1 min 52 sec
• test_multiprocessing_fork: 1 min 23 sec

1 test failed:
test_builtin

17 tests skipped:
test_devpoll test_idle test_ioctl test_kqueue test_msilib
test_ossaudiodev test_startfile test_tcl test_tix test_tk
test_ttk_guionly test_ttk_textonly test_turtle test_winconsoleio
test_winreg test_winsound test_zipfile64

1 re-run test:
test_builtin

Total duration: 30 min 44 sec

Click to see traceback logs

Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_builtin.py", line 1945, in test_input_no_stdout_fileno
    lines = self.run_child(child, b"quux\r")
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_builtin.py", line 1894, in run_child
    support.wait_process(pid, exitcode=0)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/support/__init__.py", line 3456, in wait_process
    raise AssertionError(f"process {pid} exited with code {exitcode2}, "
AssertionError: process 11103 exited with code -1, but exit code 0 is expected


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_builtin.py", line 1945, in test_input_no_stdout_fileno
    lines = self.run_child(child, b"quux\r")
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_builtin.py", line 1894, in run_child
    support.wait_process(pid, exitcode=0)
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/support/__init__.py", line 3456, in wait_process
    raise AssertionError(f"process {pid} exited with code {exitcode2}, "
AssertionError: process 12096 exited with code -1, but exit code 0 is expected