pipenv should honor system-wide manylinux override

[rittneje]

Is your feature request related to a problem? Please describe.

As per PEP 600, I can define a _manylinux.py file in the standard library (e.g., sys.path[-1]) to override the default manylinux selection that pip uses.

However, pipenv does not honor this file, meaning it can end up selecting an undesirable wheel.

Describe the solution you'd like

pipenv should honor the system-wide manylinux override by default.

Describe alternatives you've considered

I can work around this issue by placing _manylinux.py in a directory and adding that to PYTHONPATH while running pipenv install.

[matteius]

@rittneje A lot has changed in pipenv since this was opened -- I am wondering if there has been any net change regarding this or if its still an issue?