Typo and grammar fixes

[apjanke]

Some suggested typo, grammar, and style fixes.

• Fix typos
• Add missing Oxford commas
• Replace comma splices with semicolons
• Capitalize project/brand names consistently
• Consistently use "open source" instead of "open-source"
• Do not use full stops in lists whose items are sentence fragments instead of complete sentences
• Use full stops and capitalization in lists whose items are complete sentences

This PR covers the "Getting Started with Python" through "Writing Great Python Code" sections.