How to write the requirements.txt file with "--no-binary"?

Turning my comment into an answer:

pip supports reading options from requirement files. This means that a requirements file

protobuf
--no-binary=protobuf

is a valid requirements line, same as e.g. a file consisting out of a single line

protobuf --no-binary=protobuf

This means that you can also reference other requirement files, e.g.

# requirements.txt
-r test_requirements.txt
spam eggs

Note, however, that pip install -r requirements.txt is roughly equivalent to running cat requirements.txt | xargs pip, so the options are applied to the whole command and not a single line or file. For example, this file defines conflicting options:

# requirements.txt
spam --no-binary=eggs
bacon --only-binary=eggs

An attempt of installing from this requirements file will lead to an error.