re-enable all warnings by default

[nicoddemus]

From @hunse in #1190

For interest, here's the reasoning on why DeprecationWarnings are suppressed by default. It looks like unittest re-enables them; maybe pytest should do the same?

[The-Compiler]

Should we do this for 3.0 as it's backwards-incompatible but probably relatively straightforward? Or do we just recommend people to use pytest-warnings and -Wall if they want warnings?

[RonnyPfannschmidt]

i think we discussed having it in pytest-warnings for now - @fschulze might recall more details

[fschulze]

For 3.0 we should recommend pytest-warnings, which will eventually be integrated into pytest.