Tests failing due to tox config (incompatibility with current pip version?)

[rbubley]

Problem Description

All recent PRs have had failing tests due to failure to install modules.

Example of relevant portion of test output:

  ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
  pyfolio-reloaded 0+untagged.1.gcc3b9ed requires empyrical-reloaded>=0.5.8, which is not installed.
  Successfully installed numpy-1.24.1 pandas-1.5.2 python-dateutil-2.8.2 pytz-2022.7 six-1.16.0
  pyfolio-reloaded 0+untagged.1.gcc3b9ed requires ipython>=3.2.3, which is not installed.
  Removed build tracker: '/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/pip-build-tracker-wpfqwmjx'
  pyfolio-reloaded 0+untagged.1.gcc3b9ed requires matplotlib>=1.4.0, which is not installed.
  pyfolio-reloaded 0+untagged.1.gcc3b9ed requires scikit-learn>=0.16.1, which is not installed.
  pyfolio-reloaded 0+untagged.1.gcc3b9ed requires scipy>=0.14.0, which is not installed.
  pyfolio-reloaded 0+untagged.1.gcc3b9ed requires seaborn>=0.7.1, which is not installed.

[stefan-jansen]

Addressed by #25.