[deps] Update openpyxl requirement from ~=3.1.2 to ~=3.1.5

Updates the requirements on [openpyxl](https://openpyxl.readthedocs.io) to permit the latest version.

---
updated-dependencies:
- dependency-name: openpyxl
  dependency-type: direct:production
...

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

requirements.txt

@@ -1,4 +1,4 @@
 openwisp-users @ https://github.com/openwisp/openwisp-users/tarball/master
 openwisp-utils[rest] @ https://github.com/openwisp/openwisp-utils/tarball/master
 django-reversion~=5.0.4
-openpyxl~=3.1.2
+openpyxl~=3.1.5