Bump openpyxl from 3.0.7 to 3.1.5 in /server

Bumps [openpyxl](https://openpyxl.readthedocs.io) from 3.0.7 to 3.1.5.

---
updated-dependencies:
- dependency-name: openpyxl
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

server/requirements.in

@@ -1,7 +1,7 @@
 gunicorn
 honcho
 feedgen==0.7.0
-openpyxl==3.0.7
+openpyxl==3.1.5
 
 git+https://github.com/superdesk/superdesk-core.git@hotfix/2.6.8#egg=superdesk-core
 git+https://github.com/superdesk/superdesk-planning.git@hotfix/2.6.3#egg=superdesk-planning

server/requirements.txt

@@ -170,7 +170,7 @@ oauth2client==4.1.3
     # via flask-oidc-ex
 oauthlib==3.2.2
     # via requests-oauthlib
-openpyxl==3.0.7
+openpyxl==3.1.5
     # via -r requirements.in
 ordered-set==4.1.0
     # via deepdiff