The minimal polynomial of products of rational powers of integers are
computed much faster in some cases.

In master

``minimal_polynomial(nsimplify(pi, tolerance=0.000000001))``

hangs (see #2234 and issue 4014)

Now it takes a fraction of a second.
added missing import
Merge pull request #2288 from pernici/mp3

In ``_minpoly1`` used a faster algorithm for some products.