MAINT: stats: split the distribution parameters from the test suite
TST: stats: levy_stable, uncomment, skip in tests
MAINT: stats: move _construct_doc to rv_generic
DOC: stats: typos in johnsonSb, wald docstring
DOC: stats: replace examples in docstrings of distributions
MAINT: keep python -OO happy
DOC: fix issues in stats.distributions docstrings.

Now shows plots of dist.pdf() in generated docs.
BUG: stats.distributions: _drv method was broken.

Observed in doc build output, but apparently not covered by a test.
ENH: tweak plots in docstring examples