.github/workflows/tox.yml: Switch to macosx 10.15

Use macos 10.15 since they're with vagrant and virt but
don't use 11 and never due to missing vagrant on them.

Ref:
actions/runner-images#4060
actions/runner-images#4010

Signed-off-by: Arnaud Patard <apatard@hupstream.com>

Author: apatard

.github/workflows/tox.yml

@@ -31,16 +31,16 @@ jobs:
             python-version: 3.7
             unit: false
           - tox_env: py37
-            os: ubuntu-20.04
+            os: macos-10.15
             python-version: 3.7
           - tox_env: py38
-            os: ubuntu-20.04
+            os: macos-10.15
             python-version: 3.8
           - tox_env: py39
-            os: ubuntu-20.04
+            os: macos-10.15
             python-version: 3.9
           - tox_env: py310
-            os: ubuntu-20.04
+            os: macos-10.15
             python-version: "3.10"
 
     env: