improve dash import test

[alexcjohnson]

Fixes #1143 - with the help of companion PRs in the various component repos. (In principle we should do this for all component repos as well as the boilerplate)

• improve dash import test dash-core-components#903
• improve dash import test dash-html-components#169
• improve dash import test dash-table#854

Through a confusing set of circumstances described in #1143, having a Python file with one of several common names in your directory (but NOT dash.py) which imports any dash component package causes a circular import of dash itself, which manifests as failing the "make sure you don't have a dash.py file in your directory" check in the __init__.py of the component package, because dash hasn't finished importing so doesn't have the attributes we're looking for.

I'm tempted to delete these checks entirely - if we did that, the circumstance described above would work fine, but when you do make a dash.py you'd see the error message:

  File "/Users/alex/plotly/dash-core-components/dash_core_components/__init__.py", line 30, in <module>
    from ._imports_ import *  # noqa: F401, F403
  File "/Users/alex/plotly/dash-core-components/dash_core_components/_imports_.py", line 1, in <module>
    from .Checklist import Checklist
  File "/Users/alex/plotly/dash-core-components/dash_core_components/Checklist.py", line 3, in <module>
    from dash.development.base_component import Component, _explicitize_args
ModuleNotFoundError: No module named 'dash.development'; 'dash' is not a package

Which is essentially what you'd see if you did this with any other Python package - import pandas when you have a file pandas.py for example.

And if we ever end up combining packages this whole business will be moot, as you'll never get to the point of generating this error. Still, for now these changes should solve the issue.

Contributor Checklist

• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR
• I have added entry in the CHANGELOG.md

[Marc-Andre-Rivet]

after 🔪 https://github.com/plotly/dash/pull/1493/files#r539834762