Make pip download prefer to download source-distributions (not wheels)

use pip download --no-binary=:all: -r requirements.txt

According to the pip documentation:

--no-binary:

Do not use binary packages. Can be supplied multiple times, and each time adds to the existing value. Accepts either :all: to disable all binary packages, :none: to empty the set, or one or more package names with commas between them. Note that some packages are tricky to compile and may fail to install when this option is used on them.

It worked for me!

According to pip install -h:

--no-use-wheel Do not Find and prefer wheel archives when searching indexes and find-links locations. DEPRECATED in favour of --no-binary.

And

--no-binary Do not use binary packages. Can be supplied multiple times, and each time adds to the existing value. Accepts either :all: to disable all binary packages, :none: to empty the set, or one or more package

You may need to upgrade pip with pip install -U pip if your version is too old.