Update protobuf requirement from ==3.13.* to >=3.13,<3.16

Updates the requirements on [protobuf](https://github.com/protocolbuffers/protobuf) to permit the latest version.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.13.0-rc3...v3.15.6)

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

setup.py

@@ -28,7 +28,7 @@ def read(*parts):
 install_requires = [
     "click==7.1.*",
     "graphviz>=0.14,<0.17",
-    "protobuf==3.13.*"
+    "protobuf>=3.13,<3.16"
 ]
 
 setup_requires = [