How can I specify a custom Git branch in my Pipfile?

Here is some documentation on the GitHub repo for pipenv:

You can install packages with pipenv from git and other version
control systems using URLs formatted according to the following rule:

<vcs_type>+<scheme>://<location>/<user_or_organization>/<repository>@<branch_or_tag>#egg=<package_name>

So, for example:

[packages]
requests = {git = "https://github.com/requests/requests.git", editable = true, ref = "v2.20.1"}

You can generate a Pipfile using the command line. The Pipfile above was generated with:

pipenv install -e git+https://github.com/requests/requests.git@v2.20.1#egg=requests

The documentation for pip goes into more detail.