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
Fix doctest blacklist, broken by 68ddfe8
Merge pull request #2288 from pernici/mp3

In ``_minpoly1`` used a faster algorithm for some products.
Merge pull request #2602 from skirpichev/fix-doctest

Fix doctest blacklist, broken by 68ddfe8
Implement a new syntax for multivariable Order symbol

    Supported input:
    O(expr)
    O(expr, var)
    O(expr, var, pt)
    O(expr, (var1, var2, ..., varN))
    O(expr, (var1, var2, ..., varN), (pt1, pr2, ..., ptN))
Fix Order syntax in some tests
raise an error for O(f(x), x, x)
Fix tests for as_expr_variables
Fix printing tests