Fix 1021560 and 1023040

Paul Geversrequested to merge
elbrus/nbconvert:fix-RC-issues into master

Add 0007-fix-SyntaxError:_invalid_escape_sequence.patch to fix an import error in the Debian vendored _mistune.py (Closes: #1021560)

Don't fail on deprecation warning during autopkgtest

Add 0008-dont-use-intersphinx-during-build.patch to prevent downloading artifact from the internet during build (Closes: #1023040)

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