DOC: Switch to OO code style & ensure fixed y-range in `psd_demo3`

[afvincent]

In psd_demo3.py pylab example:

• adopt OO code style;
• fix the y-range to avoid discrepancies between the two subplots in the case of y-values in the 1st subplot that exceed the provided y-tick range (due to an “unfortunate” random draw) .

[efiring]

All examples such as this one that use random data should first call np.random.seed(1) (or some unsigned integer value) so that the data will be repeatable.

[afvincent]

@efiring I agree with you: the example now uses a dedicated RandomState instance as prng. I think this approach is still readable (np.random becoming simply prng here) and it avoids messing with the default prng np.random.

It is part of my plans to try to introduce this kind of reproducible (pseudo-)random draws in the relevant mpl examples (but I do not really have the bandwidth currently…)

[tacaswell]

backported to v2.x as a5fe337