Implementing pickle methods (#535)

Continuing work from #484 by @kevinaud to fix #419

* Implementing pickle methods
* Implement __copy__, __reduce__, and fix __setstate__
* Moved pickling tests into their own file
* Add test for caching

pyproject.toml

@@ -21,7 +21,7 @@ python-dateutil = "^2.8"
 isort = {version = "^5.11.5", optional = true}
 typing-extensions = "^4.7.1"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 asv = "^0.4.2"
 bpython = "^0.19"
 grpcio-tools = "^1.54.2"
@@ -39,7 +39,7 @@ tomlkit = "^0.7.0"
 tox = "^3.15.1"
 pre-commit = "^2.17.0"
 pydantic = ">=1.8.0,<2"
-
+cachelib = "^0.10.2"
 
 [tool.poetry.scripts]
 protoc-gen-python_betterproto = "betterproto.plugin:main"