Test and announce python 3.10 compatibility

Author: palango

.github/workflows/python-package.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v2

setup.py

@@ -100,6 +100,7 @@ def run(self) -> None:  # pylint: disable=no-self-use
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
     ],
     "entry_points": {"console_scripts": ["deploy = raiden_contracts.deploy.__main__:main"]},
     "cmdclass": {