Add Pharo 13 to build matrix

Author: svenvc

.github/workflows/CI.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        smalltalk: [ Pharo64-12, Pharo64-11, Pharo64-10, Pharo64-9.0, Pharo64-8.0, Pharo64-7.0 ]
+        smalltalk: [  Pharo64-13, Pharo64-12, Pharo64-11, Pharo64-10, Pharo64-9.0, Pharo64-8.0, Pharo64-7.0 ]
     name: ${{ matrix.smalltalk }}
     steps:
       - uses: actions/checkout@v2