Safer install instructions using virtualenv, more convinient client_secrets.json location, Python3 support

[DougBeney]

This pull request:

• Offers safer install instructions to use the package in a virtualenv. You should avoid using pip as sudo. (Everything still functions fine for those who do not want to use a virtualenv)
• Offers an additional location for this script to look for client_secrets.json. It also looks in [PATH YOU INSTALLED YOUTUBE UPLOAD TO]/client_secrets.json
• Fixes one error in Python3

[kootsoop]

@DougBeney Thanks for this! I'd prefer to keep both install instructions, rather than just the virtual env (which is a great addition!). Any comment?

[DougBeney]

Yeah, that's fine. More options the merrier. Of course, it would be good to label one of the methods as "recommended" for those confused on which to go with.

[agiz]

Disaster of a PR. Doesn't work, incomplete instructions /usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/Resources/Python.app/Contents/MacOS/Python: can't open file '/Users/ziga/Documents/projects/youtube-tts/yt-upload-2/build/scripts-3.7/youtube-upload.py': [Errno 2] No such file or directory

[DougBeney]

@agiz Disaster of a critique. You need to provide more info. Which command did you run that generated that?