Introduce PYSMT_SOLVER environment option
Introduce PYSMT_SOLVER environment option

Using the PYSMT_SOLVER system environment option, it is possible to
restrict the set of installed solvers that are actually accessible to
pySMT. For example, setting PYSMT_SOLVER="msat,z3" will limit the
accessible solvers to msat and z3.

This is useful for testing.

pysmt/pysmt,agriggio/pysmt,agriggio/pysmt,pysmt/pysmt