Update mistune requirement from <1 to <4 (#1306)

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/doc-requirements.txt

@@ -1,5 +1,5 @@
 # https://github.com/miyakogi/m2r/issues/66
-mistune<1
+mistune<4
 myst-parser
 pydata_sphinx_theme
 sphinx