migrate to the new QCS API client

[erichulburd]

TODO

• Install the new QCS API Client
• Drop the following file:
  • api/_config.py
• replace the following functions below
  • api/_base_connection.py
    • get_json and post_json depends on the url path, but those generic functions can be replaced with calls to the new API client.
    • we can drop parse_error and rely on the shape of https://docs.api.qcs.rigetti.com/#tag/errors.
    • we can drop get_session with contextual initialization of the QCS API client as described in usage.
    • we can drop ForestSession, ForestConnection, and Engagement
    • Any reference to engagements can instead use CreateEngagement.
  • api/_devices.py (rename to api/_quantum_processors.py.
    • list_devices, list_lattices, and get_lattice will be replaced by ListQuantumProcessors and GetInstructionSetArchitecture.
• api/_errors.py, drop the following errors and instead rely on API error messages in the new APIError
  • "device_offline": DeviceOfflineError,
  • "device_retuning": DeviceRetuningError,
  • "invalid_input": InvalidInputError,
  • "invalid_user": InvalidUserError,
  • "job_not_found": JobNotFoundError,
  • "missing_permissions": MissingPermissionsError
• device/**
  • We'll have to either parse the new ISA to the existing Device class or think about whether we want to change that interface at all.
• Drop, update, and add tests as appropriate.

Description

Insert your PR description here. Thanks for contributing to pyQuil! 🙂

Checklist

• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on Travis CI.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• All code follows Black style and obeys flake8 conventions.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.
• The changelog is updated, including author and PR number (@username, gh-xxx).

[erichulburd]

Closing in favor of #1300