Add timestamp argument to STEP export

[jgraichen]

Allow passing a timestamp value when exporting STEP files, to generate STEP files with a specific timestamp value in the file header.

For example, a null timestamp (0000-00-00T00:00:00), or a static timestamp can be used when generated files should be equal if there are no visual changes, such as for file versioning.

This commit extends the #export_step function, to accept a timestamp keyword argument, that can be a string or a datetime object. Accepting a datetime object makes it easier to use from Python, like a source file modification date.

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In jgraichen/someline I store the exported step files in the Git repository, but the current timestamp in the file always results in changes. I use them there in a CI pipeline to ensure that the code always generates the same models as before.

Currently, I have to strip the timestamp after the export. Passing a timestamp directly when exporting would make it much easier and directly allow reproducible builds.

Some formatting changes are from black, some imports were not used anymore.

[gumyr]

Thanks, this looks like a good feature. It can be irritating when things like timestamps make files artificially different.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.93%. Comparing base (7c52e50) to head (2f5bb9c).
Report is 11 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #984      +/-   ##
==========================================
- Coverage   96.94%   96.93%   -0.01%     
==========================================
  Files          32       32              
  Lines        9883     9887       +4     
==========================================
+ Hits         9581     9584       +3     
- Misses        302      303       +1     

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