Actually, if given package is not on PyPI (or you want a specific branch) you can still install it through pip from GitHub with:
pip install git+https://github.com/[repo owner]/[repo]@[branch name]
And for your problem it would be (although @pandita’s answer is correct for normal usage case):
pip install git+https://github.com/praw-dev/praw.git
For more information check this answer.