Installation problems on virtual environments with source releases

[astrojuanlu]

Describe the bug
I tried pip install . but not all subpackages are installed, for example d3rlpy.models.torch.q_functions (see readthedocs/readthedocs.org#8326).

To Reproduce

1. Create virtual environment and activate it: python3.7 -m venv .venv && source .venv/bin/activate
2. Install build dependencies: pip install numpy cython
3. Run pip install .
4. Change to a different directory (to avoid importing from the same directory, see https://hynek.me/articles/testing-packaging/#src)
5. Run import d3rlpy
6. See crash

Expected behavior
The package source code can be installed and properly imported in a virtual environment.