`isort`: First Party Modules (#4902)

* `isort`: First Party Modules

Declare `amrex` and `pywarpx` our first party modules, even if they
are not installed. This resolves inconsistencies when sorting imports
in developer or remote runs (pre-commit.com CI).

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci



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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>