You can fetch all dependencies with the following command (an Internet connection is of course required)
pip download -r requirements.txt
Then, you can install offline those dependencies with the following command
pip install -r requirements.txt --no-index --find-links file:///tmp/packages
--no-index : Ignore package index (only looking at --find-links
URLs instead).
-f, --find-links <URL> : If a URL or path to an html file, then
parse for links to archives. If a local path or file:// URL that's a directory, then look for
archives in the directory listing.
This answer is taken from this post