Raise informative error whenever context is pickled

[cicdw]

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Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

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What does this PR change?

This PR raises an informative error whenever context is pickled. Closes #1710

Why is this PR important?

Users are tempted to use context like:

from prefect import context

@task
def my_fn():
    logger = context['logger']

and submit such tasks to dask. This is problematic, because if we freeze context (which is what this import pattern effectively does), we lose all the dynamic information that is placed into context by the various runners.

Consequently, users should always access context as an attribute of the top-level module so that dynamically added keys will be available.

This PR raises an informative error for how to access context in the event that it is pickled.

[codecov]

Codecov Report

Merging #1711 into master will decrease coverage by 0.01%.
The diff coverage is 100%.