[WIP] update for Python 3.9

[jdebacker]

Updates Package-Builder to create packages for Python 3.9.

[jdebacker]

I've been unable to successfully create a package using pkgbld build with this branch or the master branch. Errors are similar. Here's what I'm seeing when I try to create a package for CCC from this branch:

Extracting package attributes for upload
Creating package "ccc"
Creating release "1.2.5"
Traceback (most recent call last):
  File "/Users/jason.debacker/repos/Package-Builder/pkgbld/utils.py", line 20, in os_call
    subprocess.run(cmd, shell=True, check=True, stdout=subprocess.PIPE)
  File "/Users/jason.debacker/anaconda3/envs/pkgbld-dev/lib/python3.7/subprocess.py", line 512, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command 'anaconda -t /Users/jason.debacker/.pslmodels_anaconda_token upload -u pslmodels --force pkgbld_output/osx-64/ccc-1.2.5-py37_0.tar.bz2' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/jason.debacker/anaconda3/envs/pkgbld-dev/bin/pbrelease", line 33, in <module>
    sys.exit(load_entry_point('pkgbld', 'console_scripts', 'pbrelease')())
  File "/Users/jason.debacker/repos/Package-Builder/pkgbld/cli.py", line 103, in main
    local=args.local, dryrun=args.dryrun)
  File "/Users/jason.debacker/repos/Package-Builder/pkgbld/release.py", line 215, in release
    u.os_call(cmd)
  File "/Users/jason.debacker/repos/Package-Builder/pkgbld/utils.py", line 25, in os_call
    raise OSError(msg.format(err.returncode, cmd, err.output))
OSError: non-zero return code 1 generated by command:
anaconda -t /Users/jason.debacker/.pslmodels_anaconda_token upload -u pslmodels --force pkgbld_output/osx-64/ccc-1.2.5-py37_0.tar.bz2
>output: b''

CC @hdoupe @MattHJensen @Peter-Metz