Could not find a version that satisfies the requirement <package>
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>
This approach (having all dependencies in a directory and not downloading from an index) only works when the directory contains all packages. The directory should therefore contain all dependencies but also all packages that those dependencies depend on (e.g., six, pytz etc).
You should therefore manually include these in requirements.txt (so that the first step downloads them explicitly) or you should install all packages using PyPI and then pip freeze > requirements.txt to store the list of all packages needed.
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: