Update pymdown-extensions requirement from >=6.3 to >=10.21.3

[dependabot]

Updates the requirements on pymdown-extensions to permit the latest version.

Release notes

Sourced from pymdown-extensions's releases.

10.21.3

• FIX: Fix regression that allows a snippet to be loaded outside of the base path using directory traversal when restrict_base_path is enabled (the default). Found by @​gistrec.

Commits

• 4262841 Fix spelling
• 63b7835 Merge commit from fork
• 3d18550 Docs: update js deps
• a4fdd73 Skip tag 10.21.1 has we accidentally already used it
• 8afb4cd Docs: Update JS deps
• 7bf5b29 Pygments needs a non-None value for code block title (#2863)
• 20b11eb Fix some spelling and formatting
• c9edba3 Docs: strengthen Snippets warning and add security considerations
• 6d92b68 Bump version
• baeca0e Docs: update JS deps
• Additional commits viewable in compare view

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