remove support for jax 0.3 / Python 3.7 (#1468)

Jax dropped support for Python 3.7 (which is officially unsupported by
the numerical ecosystem since December).
PR #1461 for a fix requires recent versions of jax >=0.4, and I don't
have the time to invest in finding workarounds to fix the bug with older
jax versions.

As we [have almost no users who install netket on python
3.7](https://pypistats.org/packages/netket) (on average we have 1
download per day, vs 35 on python 3.9 and 40, and jax already dropped
support for python 3.7, I would like to go down this way.

In any case, more modern tricks with jax.array and auto partitioning
will require it so...