Part of T1261

Prior to this, when wrong url injection happens, no information is
fetched because we have bad input in the first place (404, 400).  This
makes the debugging session quite hard.

Only the main gitlab.com instance supports a rate limit (thus the
credentials need), others do not (as per summer 2018).

related to T1242

The previous url is correct and redirects to this new one.

Related T422

Related T422

The xmlrpc is marked as deprecated [1]. Even if it's not now, the
legacy api is not marked as deprecated. So moving towards this one
sounds more reasonable [2].

[1] https://warehouse.readthedocs.io/api-reference/xml-rpc/#pypi-s-xml-rpc-methods

[2] https://warehouse.readthedocs.io/api-reference/legacy/#simple-project-api

Related T422

Based solely on pypi's deprecated xmlrpc api [1].  No other way of listing
pypi.org is referenced (except for parsing an html page through a
legacy api [2])

[1] https://warehouse.readthedocs.io/api-reference/xml-rpc/#pypi-s-xml-rpc-methods

[2] https://pypi.python.org/simple/

Related T422

That's needed for example for having the minimum necessary to make the
debian lister run.

This reverts commit e61512af.

This commit contains one section not supposed to be there yet (pypi is
not ready)

That's needed for example for having the minimum necessary to make the
debian lister run.

UnboundLocalError could happen otherwise