removing pypy-3.9 not available in gh actions

Author: madzak

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [pypy-3.6, pypy-3.7, pypy-3.8, pypy-3.9, 3.6, 3.7, 3.8, 3.9, 3.10.0-beta.4]
+        python-version: [pypy-3.6, pypy-3.7, pypy-3.8, 3.6, 3.7, 3.8, 3.9, 3.10.0-beta.4]
 
     steps:
       - uses: actions/checkout@v2