ci + test: buster, python 3.7 support

[redshiftzero]

Description

towards #527

add buster test job (build job we can't add until we update our pip mirror) and gets tests passing on buster / python 3.7.

the one application change here is to ensure that calling safe_mkdir(sdc_home) directly actually creates sdc_home - previously it would just return without creating the directory.

Testing

1. Make a python 3.7 virtualenv
2. Ensure you can start the application
3. New CI job should also pass

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

[sssoleileraaa]

lgtm!