Auto merge of #17103 - aneeshusa:shim-subprocess-devnull, r=larsbergstrom

Shim subprocess.DEVNULL for Python 2

This was introduced in Python 3.3, so provide our own version.

Requires careful review since this will leak secrets (!!!) if not done properly.

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- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes help with #17045 (github issue number if applicable).

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