Fix _processIncludes to take StringIO argument

[opotowsky]

What is the change?

Added the capability for _processIncludes to handle StringIO input for src argument.

Why is the change being made?

resolveMarkupInclusions claims to handle StringIO and TextIOBase as inputs for the src, but _processIncludes only handled the scenario where there was a file path input.

This is discussed more in #1329.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[opotowsky]

@john-science Here's the deal...

TextIOBase SHOULD in THEORY have a readlines attribute. I just couldn't figure out how to create a TextIOBase object in my code properly to test that. Which means the unit test I would add isn't going to work.

Yet?

BRB will work on this tomorrow :-)

[opotowsky]

Ok this is ready for review now @john-science and can me merged at your leisure if you feel its ready

I think the TextIOBase case is quite limited because I couldn't figure out how to load the yaml into a bytesIO object into a TextIOBase or TextIOWrapper object to be used as input here. So I'm limiting this PR to the StringIO addition.

[john-science]

Awesome!