Bump mistune from 0.8.4 to 2.0.3 in /examples/research_projects/lxmert (#18370)

Bumps [mistune](https://github.com/lepture/mistune) from 0.8.4 to 2.0.3.
- [Release notes](https://github.com/lepture/mistune/releases)
- [Changelog](https://github.com/lepture/mistune/blob/master/docs/changes.rst)
- [Commits](https://github.com/lepture/mistune/compare/v0.8.4...v2.0.3

)

---
updated-dependencies:
- dependency-name: mistune
  dependency-type: direct:production
...
Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/research_projects/lxmert/requirements.txt

@@ -40,7 +40,7 @@ kiwisolver==1.2.0
 lockfile==0.12.2
 MarkupSafe==1.1.1
 matplotlib==3.3.1
-mistune==0.8.4
+mistune==2.0.3
 msgpack==0.6.2
 nbclient==0.5.0
 nbconvert==6.0.1