Update build.yml dropping Python 2.7 (google#479)

We'll want to reenable later.

Author: dbieber

.github/workflows/build.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: ["2.7", "3.5", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.5", "3.7", "3.8", "3.9", "3.10"]
 
     steps:
      # Checkout the repo.