Do not build for macOS 10.15 (#16)

macOS 10.15 was retired on GHA

Author: iakov

.github/workflows/build.yml

@@ -140,8 +140,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-       macos-version:  ['10.15']
-       python-version: ['3.6']
+       macos-version:  ['11']
+       python-version: ['3.9']
        qt-version: ['5.12.*']
        configuration: ['release']
        include: