Traits Futures 0.2.0

This is a feature release of Traits Futures. The main features of this
release are:

• Improved support for user-defined background task types.
• Easier creation of background calculations that can be (cooperatively)
  cancelled mid-calculation.
• Significant internal refactoring and cleanup, aimed at eventual support
  for alternative front ends (GUI event loops other than the Qt event
  loop) and back ends (e.g., multiprocessing).
• Improved and expanded documentation.

There are no immediately API-breaking changes in this release: existing working
code using Traits Futures 0.1.1 should continue to work with no changes
required. However, some parts of the existing API have been deprecated, and
will be removed in a future release. See the Changes section below for more
details.

Detailed changes follow. Note that the list below is not exhaustive: many
more minor PRs have been omitted.

Features

• Users can now easily create their own background task types to supplement
  the provided task types (background calls, background iterations and
  background progress). A combination of a new ITaskSpecification
  interface and a convenience BaseFuture base class support this.
  (#198)
• The submit_iteration function now supports generator functions that
  return a result. This provides an easy way to submit background computations
  that can be cancelled mid-calculation. (#167)
• The TraitsExecutor class now accepts a max_workers argument,
  which specifies the maximum number of workers for a worker pool created
  by the executor. (#125)
• There are new task submission functions submit_call,
  submit_iteration and submit_progress. These functions replace
  the eponymous existing TraitsExecutor methods, which are now
  deprecated. (#166)
• There's a new IFuture interface class in the
  traits_futures.api module, to aid in typing and Trait declarations.
  (#169)
• A new IParallelContext interface supports eventual addition
  of alternative back ends. The new MultithreadingContext class
  implements this interface and provides the default threading back-end.
  (#149)

Changes

• The state trait on CallFuture, IterationFuture and ProgressFuture is
  now read-only. It used to be writable.
• The default number of workers in an owned worker pool (that is, a worker pool
  created by a TraitsExecutor) has changed. Previously it was
  hard-coded as 4. Now it defaults to whatever Python's
  concurrent.futures executors give, but can be controlled by passing
  the max_workers argument. (#125)
• The submit_call, submit_iteration and submit_progress
  methods on the TraitsExecutor have been deprecated. Use the
  submit_call, submit_iteration and submit_progress
  convenience functions instead. (#159)
• The thread_pool argument to TraitsExecutor has been renamed
  to worker_pool. The original name is still available for backwards
  compatibility, but its use is deprecated. (#144, #148)
• Python 2.7 is no longer supported. Traits Futures requires Python >= 3.5,
  and has been tested with Python 3.5 through Python 3.9. (#123, #130, #131,
  #132, #133, #138, #145)

Fixes

• Don't create a new MessageRouter at executor shutdown time. (#187)

Tests

• Fix some intermittent test failures due to test interactions. (#176)
• The 'null' backend that's used for testing in the absence of a Qt backend
  now uses a asyncio-based event loop instead of a custom event loop.
  (#107, #179)
• Rewrite the Qt GuiTestAssistant to react rather than polling. This
  significantly speeds up the test run. (#153)
• Ensure that all tests properly stop the executors they create. (#108, #146)
• Refactor the test structure in preparation for multiprocessing
  support. (#135, #141)
• Test the GuiTestAssistant class. (#109)

Developer tooling

• Add a new python -m ci shell click cmd. (#204)
• Update edm version in CI. (#205)
• Add checks for missing or malformed copyright headers in Python files (and
  fix existing copyright headers). (#193)
• Add import order checks (and fix existing import order bugs). (#161)
• Add separate "build" and "ci" modes for setting up the development
  environment. (#104)
• Don't pin dependent packages in the build environment. (#99)

Documentation

• Update docs to use the Enthought Sphinx Theme. (#128)
• Autogenerated API documentation is now included in the documentation
  build. (#177, #181)
• Restructure the documentation to avoid nesting 'User Guide'
  under 'User Documentation'. (#191)
• Document creation of new background task types. (#198)
• Document use of submit_iteration for interruptible tasks. (#188)