Don't default index url to public pypi. Defer to underlying libs

[analogue]

Testing done: `tox -e upgrade-requirements' now works with serviecs/ad_realtime_bidding

Internal ticket: CORESERV-10904

[ymilki]

lgtm. We want to rely on the normal expected defaults for pypi.

[asottile]

please don't change this -- it breaks my uses of this

the service in question should be fixed instead to utilize the --index-url argument explicitly

[analogue]

please don't change this -- it breaks my uses of this

the service in question should be fixed instead to utilize the --index-url argument explicitly

We'd like the default to be picked up from /etc/pip.conf which I believe is the default for the rest of python / pip related tools. Having upgrade-requirements as the the outlier is not desirable.

If you consider this a breaking change, I can bump the version accordingly.

[asottile]

A better solution would be to remove the argument entirely and release 2.0.0

please don't merge with unresolved reviews