Cannot install via pipenv

[meronym]

Pipenv is a new Python packaging tool with an emphasis on predictable dependency resolution and deterministic builds.

In its current form, pyethereum cannot be installed via Pipenv (the install process freezes indefinitely, probably at the dependency resolution stage). I haven't fully investigated the issue as the dependency graph looks too complex for me to debug, but related comments in the Pipenv community point out that this might be caused by dependency versioning conflicts.

Steps to reproduce:

pipenv install git+https://github.com/ethereum/pyethereum.git
Creating a virtualenv for this project…
Using /usr/local/Cellar/pipenv/2018.5.18/libexec/bin/python3.6 (3.6.5) to create virtualenv…
Installing setuptools, pip, wheel...done.

Creating a Pipfile for this project…
Installing git+https://github.com/ethereum/pyethereum.git…

(cannot move past this step, frozen indefinitely) 

[cleanunicorn]

Install works with pip

$ pip install git+https://github.com/ethereum/pyethereum.git --user
Collecting git+https://github.com/ethereum/pyethereum.git
  Cloning https://github.com/ethereum/pyethereum.git to /tmp/pip-req-build-j3r5c_qi
...
Successfully built ethereum

And you can import and use it

$ ipython       
Python 3.7.2 (default, Jan 10 2019, 23:51:51) 
Type 'copyright', 'credits' or 'license' for more information
IPython 6.5.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import ethereum

In [2]: from ethereum import vm

In [3]: vm.preprocess_code(bytes.fromhex("6020"))
Out[3]: (0, {0: 32})