Add Py3.10 as CI test target.

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
         # in all python versions and test failures (builtin_float) in 3.5<
         os: [ubuntu-18.04]
         backend: [c, cpp]
-        python-version: [2.7, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 3.10-dev]
+        python-version: ["2.7", "3.4", "3.5", "3.6", "3.7", "3.8", "3.9", "3.10"]
         env: [{}]
 
         include:
@@ -195,6 +195,14 @@ jobs:
             python-version: 3.9
             backend: cpp
             env: { MACOSX_DEPLOYMENT_TARGET: 10.14 }
+          - os: macos-10.15
+            python-version: "3.10"
+            backend: c
+            env: { MACOSX_DEPLOYMENT_TARGET: 10.14 }
+          - os: macos-10.15
+            python-version: "3.10"
+            backend: cpp
+            env: { MACOSX_DEPLOYMENT_TARGET: 10.14 }
 
     # This defaults to 360 minutes (6h) which is way too long and if a test gets stuck, it can block other pipelines.
     # From testing, the runs tend to take ~20 minutes, so a limit of 30 minutes should be enough. This can always be