Bump protobuf from 3.9.1 to 3.15.0 (#41)

* Bump protobuf from 3.9.1 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.9.1 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.9.1...v3.15.0)

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

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

* Make protobuf version flexible

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Pierre R. Mai <pmai@pmsf.de>

Author: dependabot[bot]

setup.py

@@ -50,7 +50,7 @@
             "colorama",
             "tabulate",
             "progress",
-            "protobuf==3.9.1",
+            "protobuf>=3.15.0",
         ],
         dependency_links=[
             "git+https://github.com/OpenSimulationInterface/"