gh-132983: Make _zstd C code PEP 7 compliant

[emmatyping]

Mostly just making sure things are under 80 characters and fixing a few incorrectly placed left braces. I also removed const from instances of _zstd_state* const mod_state since a) it was making some lines go over 80 characters b) it is unnecessary and c) it better matches the CPython style.

This is the final todo left in #132983 🎉

• Issue: Implement PEP 784 - Adding Zstandard to the Python standard library #132983

[miss-islington-app]

Thanks @gpshead for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

3.14 branch.

[bedevere-bot]

⚠️⚠️ Buildbot failure ⚠️⚠️

Hi! The buildbot AMD64 Windows Server 2022 NoGIL 3.x (tier-1) has failed when building commit 973b8f6.

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/#/builders/1241/builds/5870) and take a look at the build logs.
4. Check if the failure is related to this commit (973b8f6) 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/#/builders/1241/builds/5870

Failed tests:

• test_weakref
• test_venv

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

==

Click to see traceback logs

Traceback (most recent call last):


Traceback (most recent call last):
  File "C:\Users\Administrator\buildarea\3.x.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\test\test_venv.py", line 616, in test_multiprocessing
    out, err = check_output([self.envpy(real_env_dir=True), '-c',
               ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        'from multiprocessing import Pool; '
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        'pool = Pool(1); '
        ^^^^^^^^^^^^^^^^^^
        'print(pool.apply_async("Python".lower).get(3)); '
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        'pool.terminate()'])
        ^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Administrator\buildarea\3.x.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\test\test_venv.py", line 58, in check_output
    raise subprocess.CalledProcessError(
        p.returncode, cmd, out, err)
subprocess.CalledProcessError: Command '['C:\\Users\\Administrator\\AppData\\Local\\Temp\\test_python_oln4cn7v\\tmp5x1oqj4y\\Scripts\\python3.15t_d.exe', '-c', 'from multiprocessing import Pool; pool = Pool(1); print(pool.apply_async("Python".lower).get(3)); pool.terminate()']' returned non-zero exit status 1.

[emmatyping]

Hm, that buildbot failure looks unrelated I think.