pipenv update --outdated --pre wont work if there are prerelease packages

[jonbesga]

Issue description

Describe the issue briefly here.

pipenv update --outdated --pre

Warning: Your dependencies could not be resolved. You likely have a mismatch in your sub-dependencies.
  You can use $ pipenv install --skip-lock to bypass this mechanism, then run $ pipenv graph to inspect the situation.
Could not find a version that matches black
Tried: 18.3a0, 18.3a0, 18.3a1, 18.3a1, 18.3a2, 18.3a2, 18.3a3, 18.3a3, 18.3a4, 18.3a4, 18.4a0, 18.4a0, 18.4a1, 18.4a1, 18.4a2, 18.4a2, 18.4a3, 18.4a3, 18.4a4, 18.4a4, 18.5b0, 18.5b0, 18.5b1, 18.5b1, 18.6b0, 18.6b0, 18.6b1, 18.6b1, 18.6b2, 18.6b2, 18.6b3, 18.6b3, 18.6b4, 18.6b4, 18.9b0, 18.9b0, 19.3b0, 19.3b0