FlexGet / Flask CORS fails to build

[sjau]

Describe the bug
FlexGet fails to build on latest git master due to Flask CORS failing.

To Reproduce

  File "/nix/store/d34icmvx8dfjsmj6ihwi8jx8kilbcmgw-python3.7-Flask-1.1.1/lib/python3.7/site-packages/flask/app.py", line 1949, in full_dispatch_request                            
    rv = self.dispatch_request()                                                                                                                                                    
  File "/nix/store/d34icmvx8dfjsmj6ihwi8jx8kilbcmgw-python3.7-Flask-1.1.1/lib/python3.7/site-packages/flask/app.py", line 1935, in dispatch_request                                 
    return self.view_functions[rule.endpoint](**req.view_args)                                                                                                                      
  File "/build/Flask-Cors-3.0.8/tests/decorator/test_exception_interception.py", line 30, in test_acl_uncaught_exception_500                                                        
    raise Exception("This could've been any exception")                                                                                                                             
Exception: This could've been any exception                                                                                                                                         
--------------------- >> end captured logging << ---------------------                                                                                                              

----------------------------------------------------------------------                                                                                                              
Ran 89 tests in 4.398s                                                                                                                                                              

FAILED (failures=1)                                                                                                                                                                 
Test failed: <unittest.runner.TextTestResult run=89 errors=0 failures=1>                                                                                                            
error: Test failed: <unittest.runner.TextTestResult run=89 errors=0 failures=1>                                                                                                     
building '/nix/store/91aqwjfwsb84lk6ghqdk3kbd4vwbx1ds-unit-zfs-zed.service.drv'...                                                                                                  
builder for '/nix/store/zj17x0n3id8qb48wzdb42i5qd4w3i799-python3.7-Flask-Cors-3.0.8.drv' failed with exit code 1                                                                    
cannot build derivation '/nix/store/g2l4z08kz4psfnzmy4m9l0zzpa2gpv77-FlexGet-2.21.25.drv': 1 dependencies couldn't be built 

Metadata
Please run nix run nixpkgs.nix-info -c nix-info -m and paste the result.

• system: "x86_64-linux"
• host os: Linux 4.19.79, NixOS, 20.03.git.4b0508a (Markhor)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.1
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos
• channels(root): "nixos-19.09pre192418.e19054ab3cd"

Maintainer information:

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# a list of nixos modules affected by the problem
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