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Integration tests are automatically skipped if TEST_CDSTAR is not defined.
Unit tests use the `responses` library to mock REST-calls and do not need
a running CDSTAR server.

As a side note, I do not think that mocking CDSTAR on REST level
is reasonable. The CDStar class should be tested against the real thing
(integration tests) and all other classes should be tested against a
mocked CDStar instance. Or, switch to a proper urllib mocking framework.
The current tests are very error prone and do not actually verify the
correctness of the client library.

Massive rewrite of the CLI and lots of refactoring work.

See merge request !8

- Changed the concept of CLI context and workspaces (see README).
- Moved 'pycdstar3.client' module to 'pycdstar3.api'.
- Moved data classes from 'pycdstar3.api' to 'pycdstar3.model'.
- Introduced optional high-level API handle classes for remote resources.

Return FileDownload from CDStar.get_file

See merge request !6

py3.5 compatibility

See merge request !7

`pkgutil.iter_modules` yielding `namedtuple`s `ModuleInfo` is new
in 3.6. On 3.5 it yields simple tuples.