GHA: Use macos-10.15 to fix build

[hugovk]

Changes proposed in this pull request:

• This repo has switched from macos-10.15 to macos-11 for macos-latest, but it's failing the build
• The quick fix to get the CI back to green is to explicitly use macos-10.15

---

For reference, here's logs from the last pass and first fail:

• last pass: logs_16004.zip
• first fail: logs_16005.zip

Last pass:

2021-12-12T09:46:43.0437590Z Processing dependencies for Pillow==9.0.0.dev0
2021-12-12T09:46:43.0460460Z Finished processing dependencies for Pillow==9.0.0.dev0

First fail:

2021-12-13T22:19:37.7945510Z Processing dependencies for Pillow==9.0.0.dev0
2021-12-13T22:19:37.7949620Z Searching for Pillow==9.0.0.dev0
2021-12-13T22:19:37.7951660Z Reading https://pypi.org/simple/Pillow/
2021-12-13T22:19:40.9891760Z No local packages or working download links found for Pillow==9.0.0.dev0
2021-12-13T22:19:40.9903990Z error: Could not find suitable distribution for Requirement.parse('Pillow==9.0.0.dev0')
2021-12-13T22:19:40.9974850Z make: *** [install-coverage] Error 1
2021-12-13T22:19:40.9991600Z ##[error]Process completed with exit code 2.

[radarhere]

Thanks

[radarhere]

I've created pypa/setuptools#2929 to ask setuptools about this problem.