chore(deps): update dependency markdown to v3.8

[slack-oss-bot]

This PR contains the following updates:

Package	Update	Change
Markdown (changelog)	minor	==3.7 -> ==3.8

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Release Notes

Python-Markdown/markdown (Markdown)

v3.8

Compare Source

Changed

• DRY fix in abbr extension by introducing method create_element (#​1483).
• Clean up test directory by removing some redundant tests and port
  non-redundant cases to the newer test framework.
• Improved performance of the raw HTML post-processor (#​1510).

Fixed

• Backslash Unescape IDs set via attr_list on toc (#​1493).
• Ensure md_in_html processes content inside "markdown" blocks as they are
  parsed outside of "markdown" blocks to keep things more consistent for
  third-party extensions (#​1503).
• md_in_html handle tags within inline code blocks better (#​1075).
• md_in_html fix handling of one-liner block HTML handling (#​1074).
• Ensure <center> is treated like a block-level element (#​1481).
• Ensure that abbr extension respects AtomicString and does not process
  perceived abbreviations in these strings (#​1512).
• Ensure smarty extension correctly renders nested closing quotes (#​1514).

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