Cuda request gpu device expr (common-workflow-language#1629)

* Allow expressions to dynamically request min/max number of GPUs.

* Add test coverage for CUDA checks

Fix a few help texts

The continued strings on a separate line just get ignored by the runtime. You need to put them in parentheses to turn them into a single string.

Allow records with defaults to be accepted, also type longs

Revert "Drop Python 3.6" (common-workflow-language#1620)

* Revert "Drop Python 3.6"

Ubuntu 18.04 (LTS, supported until April 2023) and Centos 7 are still
on Python 3.6.

This reverts commit 780f64a.

* Review fixes and run 'make sort_imports'

scandeps should not apply mergedirs at every level (common-workflow-l…

…anguage#1615)

* scandeps should not apply mergedirs at every level

When getting back a list of dependencies, and using "nestdirs", in the
specific case it there are references to both a directory and files
within that directory, it may report multiple dependencies for the
same directory.  The mergedir method merges those references into one.

* Add comments to scandeps

* Add test.  Fix mypy/format

improve tests

add missing @needs_docker

Better errors for expressions in input format fields

single-process: handle step-level req collisions

test_bad_stdout_expr_error needs docker