Construct CartAPI URL from OS environment variables

[markborkum]

Description

This pull request modifies the constructors for the Downloader and CartAPI classes so that CartAPI URLs are automatically constructed from either user-specified keyword arguments or OS environment variables.

The intent is that the type signature for the constructor for the Downloader class should resemble that of the Uploader (e.g., use keyword arguments to specify and/or construct URLs).

Changes:

• Added CommonBase class using code from https://github.com/pacifica/pacifica-python-uploader/blob/master/pacifica/uploader/common/__init__.py.
• Removed auth property from Downloader class. For a given instance d, use d.cart_api._auth instead of d.auth.

Issues Resolved

n/a

Check List

• All tests pass. See https://github.com/pacifica/pacifica-docs/blob/master/development/TESTING.md
• New functionality includes testing.
• New functionality has been documented in the README if applicable

[markborkum]

@dmlb2000 In https://github.com/pacifica/pacifica-python-downloader/pull/15/files#diff-d8e165b4149566ddfdab844d646f4d6fR51, the prefix for environment variables for the CartAPI class is "CART". Is this correct?

[dmlb2000]

@markborkum We've been standardizing on CARTD

[markborkum]

@dmlb2000 Also, should we take the opportunity to add more LOGGER.debug(...) statements to the CartAPI class (e.g., the setup_cart and wait_for_cart instance methods)?

[dmlb2000]

@markborkum Yes more logging the better really.

[markborkum]

@dmlb2000 CI is failing due to autopep8. Any ideas?

[dmlb2000]

@markborkum Yes, autopep8 is kinda lame, I would look for long lines and split them up...

foo(a, b, c, d, e)

becomes

foo(
    a,
    b,
    c,
    d,
    e,
)

[markborkum]

@dmlb2000 Ready for review.