Fix Python 3.10 test build (#228)

Author: asvetlov

.github/workflows/ci.yml

@@ -76,7 +76,7 @@ jobs:
     needs: [lint]
     strategy:
       matrix:
-        pyver: [3.6, 3.7, 3.8, 3.9, 3.10]
+        pyver: ['3.6', '3.7', '3.8', '3.9', '3.10']
         no-extensions: ['', 'Y']
         os: [ubuntu, macos, windows]
         exclude: