Pin protobuf version as it conflicts with ray

This commit can be revert as soon as
ray-project/ray#25282 is fixed.

Fix issue #192.

pyproject.toml

@@ -63,12 +63,13 @@ networkx = {version = "^2.5", optional = true}
 minizinc = {version = "^0.4.2", optional = true}
 mip = {version = "^1.13.0", optional = true}
 deap = {version = "^1.3.1", optional = true}
+protobuf = {version = "<4", optional = true}
 
 [tool.poetry.extras]
 domains = [ "gym", "numpy", "matplotlib", "simplejson" ]
-solvers = [ "gym", "numpy", "joblib", "ray", "stable-baselines3" ]
+solvers = [ "gym", "numpy", "joblib", "ray", "protobuf", "stable-baselines3" ]
 discrete_optimization = [ "networkx", "minizinc", "mip", "deap" ]
-all = [ "gym", "numpy", "matplotlib", "simplejson", "joblib", "ray", "stable-baselines3", "networkx", "minizinc", "mip", "deap" ]
+all = [ "gym", "numpy", "matplotlib", "simplejson", "joblib", "ray", "protobuf", "stable-baselines3", "networkx", "minizinc", "mip", "deap" ]
 
 [tool.poetry.plugins."skdecide.domains"]
   GymDomain = "skdecide.hub.domain.gym:GymDomain [domains]"