Force strings for GHA version matrices

Ensure that the versions listed in the version matrices used by the `test` and
`build` jobs are interpreted as strings. Without this change the version `3.10`
is interpreted as `3.1` when converted to a fixnum. Since these are intended to
be interpreted as string values we should enforce it for all versions listed.

.github/workflows/build.yml

@@ -111,11 +111,11 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - 3.6
-          - 3.7
-          - 3.8
-          - 3.9
-          - 3.10
+          - "3.6"
+          - "3.7"
+          - "3.8"
+          - "3.9"
+          - "3.10"
     steps:
       - uses: actions/checkout@v2
       - id: setup-python
@@ -197,11 +197,11 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - 3.6
-          - 3.7
-          - 3.8
-          - 3.9
-          - 3.10
+          - "3.6"
+          - "3.7"
+          - "3.8"
+          - "3.9"
+          - "3.10"
     steps:
       - uses: actions/checkout@v2
       - id: setup-python