21.0.0rc0

First official release after migrating the repository into the *NiPreps*' organization.
This release updates the Docker image with up-to-date dependencies, updates
*MRIQC*'s codebase to the latest *NiTransforms* and includes some minor bugfixes.
Finally, this release also contains a major code style overhaul by Zvi Baratz (@zvibaratz).
With thanks to Zvi Baratz, Nikita Beliy, Michael (@octomike), Benjamin Kay, Adam Huffman,
and Ursula Tooley for their contributions.

  * FIX: Revise building documentation at RTD after migration (#935)
  * FIX: Final touch-ups in the maintenance of Docker image + CI (#928)
  * FIX: Update unit tests (#927)
  * FIX: Update dependencies and repair BOLD workflow accordingly (#926)
  * FIX: Update dependencies and repair T1w workflow accordingly (#925)
  * FIX: Set ``matplotlib`` on ``Agg`` output mode (#892)
  * ENH: Refactored long strings to a :mod:`mriqc.messages` module (#901)
  * ENH: Refactored :mod:`mriqc.interfaces.common` module (#901)
  * DOC: Various fixes to "Running mriqc" section (#897)
  * MAINT: Revise Docker image settings & CircleCI (#937)
  * MAINT: Finalize transfer to ``nipreps`` organization (#936)
  * MAINT: Relicensing to Apache-2.0, for compliance with *NiPreps* and prior transfer to the org (#930)
  * MAINT: New Docker layer caching system of other *NiPreps* (#929)
  * MAINT: Code style overhaul (#901)
  * MAINT: Update ``Dockerfile`` and catch-up with *fMRIPrep*'s (#924)
  * STY: Run ``black`` at the top of the repo (#932)