Gh-12: Clean up inconsistent use of quotes in gafferpy

[wb36499]

Replaces single quotations with double quotes for strings, bracket notation & documentation strings.

[CLAassistant]

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[GCHQDeveloper314]

For context, previously there were 1073 instances of single quotes and 4462 instances of double quotes (found using grep -ro \' src | wc -l). I'm not sure you'd ordinarily want to use double quotes by default in Python, but there's no hard rule for it and given that single quote was less common I think it makes sense to replace them with double quotes.

[wb36499]

For context, previously there were 1073 instances of single quotes and 4462 instances of double quotes (found using grep -ro \' src | wc -l). I'm not sure you'd ordinarily want to use double quotes by default in Python, but there's no hard rule for it and given that single quote was less common I think it makes sense to replace them with double quotes.

I should have added that in the PR but that was essentially my rationale as well as changing any doc strings to match the PEP 8 recommendations.