GitHub Actions: Add Python 3.13 beta 4 to the build-pipeline

[cclauss]

• https://www.python.org/download/pre-releases

  • We strongly encourage maintainers of third-party Python projects to test with 3.13 during the beta phase

• https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#allow-pre-releases
• GitHub Actions: Add Python 3.13 beta 4 to the testing xhtml2pdf/xhtml2pdf#775
• Blocked by Test on 3.13-dev pyca/cryptography#9722

Traceback (most recent call last):
    from pyhanko.pdf_utils.incremental_writer import IncrementalPdfFileWriter
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko/pdf_utils/incremental_writer.py", line 8, in <module>
    from . import generic, misc
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko/pdf_utils/generic.py", line 22, in <module>
    from .misc import (
    ...<11 lines>...
    )
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko/pdf_utils/misc.py", line 17, in <module>
    from pyhanko_certvalidator.util import CancelableAsyncIterator, ConsList
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko_certvalidator/__init__.py", line 8, in <module>
    from .context import ValidationContext
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko_certvalidator/context.py", line 13, in <module>
    from .fetchers.requests_fetchers import RequestsFetcherBackend
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko_certvalidator/fetchers/requests_fetchers/__init__.py", line 8, in <module>
    from .cert_fetch_client import RequestsCertificateFetcher
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko_certvalidator/fetchers/requests_fetchers/cert_fetch_client.py", line 7, in <module>
    from ...errors import CertificateFetchError
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/pyhanko_certvalidator/errors.py", line 6, in <module>
    from cryptography.exceptions import InvalidSignature
  File "/opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/cryptography/exceptions.py", line 9, in <module>
    from cryptography.hazmat.bindings._rust import exceptions as rust_exceptions
ImportError: /opt/hostedtoolcache/Python/3.13.0-beta.4/x64/lib/python3.13/site-packages/_cffi_backend.cpython-313-x86_64-linux-gnu.so: undefined symbol: _PyErr_WriteUnraisableMsg

Description of the changes

Describe the changes in this PR, with references to issues in the issue tracker and forum discussions (if applicable).

Caveats

If the implementation has "sharp edges" or is otherwise incomplete, please explain here. Deviations from the contribution guidelines should also be motivated in this section.

In particular, if your contribution includes any of the following, please provide a brief justification here:

• Changes to project dependencies.
• Changes to existing public APIs (particularly if they could break existing user code) or the CLI.
• Nontrivial changes to internal (non-public) APIs

Checklist

Please go over this checklist to increase the chances of your PR being worked on in a timely manner. Deviations are allowed with proper justification (see previous section).

• I have read the project's CoC and contribution guidelines.
• I understand and agree to the terms in the Developer Certificate of Origin as applied to this contribution.
• All new code in this PR has full test coverage.

For new features (delete if not applicable)

• I have discussed the implementation of this feature with the project maintainer(s) on the discussion forum or over email.
• I have verified that my changes do not break existing API or CLI functionality or ensured that all breaking changes are clearly documented in this PR.
• All public API functionality in this PR is documented.

For bug fixes (delete if not applicable)

• My changes do not affect any public API or CLI semantics.
• My PR adds regression tests (i.e. tests that fail if the bug fix is not applied).
• All new code in this PR has full test coverage.

For documentation contributions (delete if not applicable)

• I have built the HTML documentation locally, and verified that the changes behave correctly in-browser.

Additional comments

Feel free to add any additional comments here.

[MatthiasValvekens]

I see that the CFFI thing has been sorted... let's give this a spin :)

[MatthiasValvekens]

Alright, lgtm. Will merge this.

(Thanks for bringing this up and looking into potential upstream blockers, by the way!)