requirement: bump mjml-python from 1.3.3 to 1.3.4 in /back

Bumps [mjml-python](https://github.com/mgd020/mjml-python) from 1.3.3 to 1.3.4.
- [Release notes](https://github.com/mgd020/mjml-python/releases)
- [Commits](mgd020/mjml-python@v1.3.3...v.1.3.4)

---
updated-dependencies:
- dependency-name: mjml-python
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

back/requirements/base.txt

@@ -11,7 +11,7 @@ djangorestframework==3.15.2
 furl==2.1.3
 hiredis==3.0.0
 humanize==4.11.0
-mjml-python==1.3.3
+mjml-python==1.3.4
 model-bakery==1.20.0
 osm_opening_hours==0.1.1
 psycopg==3.2.3