Replace custom pip freeze script with pip-tools

In the past we've avoided using out-of-the-box solutions for Python
dependency resolution because a) they haven't been very mature and b)
we've had lots of issues with version conflicts. See [[1]], [[2]] for
details. Instead, we've been using a custom Python script that
under-the-hood runs `pip freeze` and saves the output to
`requirements.txt`.

This script works well for us, but it doesn't integrate well with other
tools. On the other hand [`pip-tools`](https://github.com/jazzband/pip-tools)
as of 2020 seems to be well-supported by its maintainers and other
tools; for instance, GitHub's automated update service
[Dependabot](https://dependabot.com) supports `requirements.in` files.

This commit replaces our `freeze-requirements` make command with
`pip-compile`.

The Digital Marketplace team have made this change and seem happy with
the results.

scripts/run_tests.sh

@@ -24,9 +24,6 @@ if [[ -z "$VIRTUAL_ENV" ]] && [[ -d venv ]]; then
   source ./venv/bin/activate
 fi
 
-make test-requirements
-display_result $? 1 "Requirements check"
-
 flake8 .
 display_result $? 1 "Code style check"