DEV: Review pinned requirements in include files

[stefan6419846]

We should review the following pinned dependencies in our include files for the requirements.txt files to ensure that we keep being compatible with recent Python versions:

• Recent fpdf versions pass CI as well:

  pypdf/requirements/ci.in

  Line 6 in 50bb7f2

  fpdf2==2.4.1

• This seems to be pinned as version 2.18.0 dropped support for Python 3.6?

  pypdf/requirements/dev.in

  Line 4 in 50bb7f2

  pre-commit<2.18.0

• Enforces a specific Sphinx version? Recent versions work as well, although probably related to myst_parser:

  pypdf/requirements/docs.in

  Line 2 in 50bb7f2

  sphinx_rtd_theme<2.0.0

• myst_parser broke at some point and generated invalid links (Links to documentation pages are broken #1569). This still seems to be the case.

  pypdf/requirements/docs.in

  Line 3 in 50bb7f2

  myst_parser==0.16.1

[stefan6419846]

The myst_parser issue seems to be related to

pypdf/docs/conf.py

Line 44 in 4bdca16

myst_all_links_external = True

Changing the setting, using myst_heading_anchors = 3 and fixing the links in

docs/dev/documentation.md:56: WARNING: 'myst' cross-reference target not found: 'intro.html#commit-messages' [myst.xref_missing]
docs/user/extract-text.md:41: WARNING: 'myst' cross-reference target not found: '../modules/PageObject.html#pypdf._page.PageObject.extract_text' [myst.xref_missing]
docs/user/merging-pdfs.md:52: WARNING: 'myst' cross-reference target not found: '../modules/PdfWriter.html#pypdf.PdfWriter.append' [myst.xref_missing]
docs/user/robustness.md:32: WARNING: 'myst' cross-reference target not found: '../modules/PdfReader.md' [myst.xref_missing]
docs/user/robustness.md:33: WARNING: 'myst' cross-reference target not found: '../modules/PdfWriter.md' [myst.xref_missing]

should resolve it. This additionally requires regenerating the docs.txt requirements file afterwards.