Could not find a version that satisfies the requirement <package>

Although it doesn't really answers this specific question. Others got the same error message with this mistake.

For those who like me initial forgot the -r: Use pip install -r requirements.txt the -r is essential for the command.

The original answer:

https://stackoverflow.com/a/42876654/10093070

I had installed python3 but my python in /usr/bin/python was still the old 2.7 version

This worked (<pkg> was pyserial in my case):

python3 -m pip install <pkg>