Use pip-compile to manage requirements

[Valian]

It makes it obvious what are the "real" requirements and makes it easy to manage them. It's my standard approach in all projects and seems to be better than requirements_freeze.py (because it's widely used and maintained by other people)

https://github.com/jazzband/pip-tools

Example

# requirements.in
django

run pip-compile requirements.in:

$ pip-compile requirements.in
$ cat requirements.txt
#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile requirements.in
#
asgiref==3.2.3    # via django
django==3.0.3    # via -r requirements.in
pytz==2019.3    # via django
sqlparse==0.3.0    # via django

It will produce requirements.txt, with all the Django dependencies
(and all underlying dependencies) pinned.