Python 3.13 support

[Gallaecio]

Installing on Python 3.13 (both alpha 1 and alpha 2) currently fails as follows:

$ python3.13 -m venv venv
$ . venv/bin/activate
$ pip install zstandard
Collecting zstandard
  Downloading zstandard-0.22.0.tar.gz (660 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 660.7/660.7 kB 8.2 MB/s eta 0:00:00
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [27 lines of output]
      <string>:37: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
      <string>:38: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
      Traceback (most recent call last):
        File "/home/user/venv/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 353, in <module>
          main()
        File "/home/user/venv/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 335, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/home/user/venv/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 118, in get_requires_for_build_wheel
          return hook(config_settings)
                 ^^^^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/setuptools/build_meta.py", line 355, in get_requires_for_build_wheel
          return self._get_build_requires(config_settings, requirements=['wheel'])
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/setuptools/build_meta.py", line 325, in _get_build_requires
          self.run_setup()
        File "/tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/setuptools/build_meta.py", line 507, in run_setup
          super(_BuildMetaLegacyBackend, self).run_setup(setup_script=setup_script)
        File "/tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/setuptools/build_meta.py", line 341, in run_setup
          exec(code, locals())
        File "<string>", line 106, in <module>
        File "/tmp/pip-install-uzfqhs9l/zstandard_ed682faacb514d2a849026131c25d661/make_cffi.py", line 161, in <module>
          ffi = cffi.FFI()
                ^^^^^^^^^^
        File "/tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/cffi/api.py", line 48, in __init__
          import _cffi_backend as backend
      ImportError: /tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/_cffi_backend.cpython-313-x86_64-linux-gnu.so: undefined symbol: _PyThreadState_UncheckedGet
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

[notice] A new release of pip is available: 23.2.1 -> 23.3.1
[notice] To update, run: pip install --upgrade pip
$

[wRAR]

 ImportError: /tmp/pip-build-env-zejqvvc_/overlay/lib/python3.13/site-packages/_cffi_backend.cpython-313-x86_64-linux-gnu.so: undefined symbol: _PyThreadState_UncheckedGet

Looks like a problem with cffi.

[graingert]

fixed in python-cffi/cffi#24 but not released yet