One way to accomplish this is using a bootstrap action. You can use these to run shell scripts.
If you have a setup python file that does something like:
requirements = open("requirements.txt", "r")
shell_script = open("pip.sh", "w+")
shell_script.write("sudo apt-get install python-pip\n")
for line in requirements:
shell_script.write("sudo pip install -I " + line)
Then you can just run this as the bootstrap action without needing to upload your requirements.txt