Platform independence + some improvements

[JLanda91]

• Reformatted the file to PEP standards
• refactored path joins to work on any platform
• Fixed BeautifulSoup warning on urlopen

[JLanda91]

More improvements:

• url validation (use startswith BASE_URL)
• use f-strings
• Deleted first dir exists check, since the second will create all intermediate folders if needed

[jmc718]

This fixed OS errors I was having.

Python WindowsError: [Error 123] The filename, directory name, or volume label syntax is incorrect:

Many thanks!