Adjust the implprefix DeprecationWarning's stack level

[edmorley]

Hi!

In #144 a deprecation warning was added for usage of the implprefix argument.

However the warnings.warn() call doesn't set a stacklevel, which means when the warning is output, the blamed line is in pluggy, rather than the plugin that's using implprefix. For example:
https://travis-ci.org/mozilla/treeherder/jobs/409417404#L895

Passing stacklevel=2 would make it clearer as to where the issue lies, and also mean the correct module path can be referenced in eg pytest's filterwarnings (for downstream projects who enable warnings/deprecation warnings as errors).

Many thanks :-)

CC @tgoodlet

[nicoddemus]

Thanks @edmorley. Perhaps you would like to open a PR with that change?

[goodboy]

Yeah please do change it if it will help with the deprecation process 👍

[goodboy]

@nicoddemus should we get this one in for 0.8.0? I can PR it up if you want.

[nicoddemus]

@nicoddemus should we get this one in for 0.8.0? I can PR it up if you want.

Oh sorry, missed this one in time for 0.8.0, but no matter, we can issue a new release later.