Use ${Python3_EXECUTABLE} instead of shebang on top of Python files

[quantumsteve]

Please review the developer documentation
on the wiki of this project that contains help and requirements.

Proposed changes

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This removes the shebang on top of Python files. CMake was updated to use ${Python3_EXECUTABLE}.

I avoided files in the nexus sub-directory.

Fixes #3680

What type(s) of changes does this code introduce?

Delete the items that do not apply

• Refactoring (no functional changes, no api changes)
• Build related changes

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Checklist

Update the following with a yes where the items apply. If you're unsure about any of them, don't hesitate to ask. This is
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• Yes. This PR is up to date with current the current state of 'develop'

[prckent]

There are a lot of utilities and examples touched here that are not run by CMake. Many of the same considerations as for Nexus apply. However there is an argument for requiring the "correct" python to have to be specified by the user.

[ye-luo]

examples, labs and utils need to keep the shebang as they are not invoked by CMake/CTest.

[markdewing]

Keeping all the shebangs would be a reasonable (and safe) option. Removing them all seems like a temporary hack to find all places python scripts are called from cmake to address #3680.

[jtkrogel]

I think keeping she-bangs in anything a user might invoke is desirable. If the default provided by the she-bang is somehow wrong for their environment, they can always invoke the_correct_python_exe script. This is just about sensible defaults and convenience.

[prckent]

List of modified files LGTM

[ye-luo]

Test this please