Patch by Martin Richard.  GH PR #365.

Patch by Richard Walker.

Patch by Manuel Kaufmann.

* tracing in the "u" mode now works
* trace_vdelete() with wrong mode no longer break tracing
* trace_vinfo() now always returns a list of pairs of strings

lazy-loaded module was already in sys.modules.

dictproxy was used in Python 2 and it was renamed to mappingproxy
in Python 3.3.

Patch by Julien.

Patch by Matt Morrison.

Several test cases in distutils use PyPIRCCommandTestCase as
their base class and as a result of that the following tests
were ran more than once:

* test_server_registration
* test_server_empty_registration
* test_config_interpolation

This commit moves the infrastructure used by other tests
into a new BasePyPIRCCommandTestCase class.

Patch by Yoni Lavi

Rest of patch that should not be cherry picked into 3.5.2 final.

I noticed that there is no test for this feature while doing
triaging work on pypa/pypi-legacy.