Fix weighted term order construction

[bruno]

Currently, weighted term orders that are not given a tuple or a list as parameter behave strangely (and this causes confusion, cf ​on Ask¹):

sage: t = TermOrder('wdegrevlex', 2)
sage: t
Weighted degree reverse lexicographic term order
sage: t.is_weighted_degree_order()
False
sage: P.<x> = PolynomialRing(QQ, 1, order=t)
sage: x.degree()
1

In this ticket, we change the behavior as follows:

sage: t = TermOrder('wdegrevlex', 2)
sage: t
Weighted degree reverse lexicographic term order with weights (2,)
sage: t.is_weighted_degree_order()
True
sage: P.<x> = PolynomialRing(QQ, 1, order=t)
sage: x.degree()
2

Note: Another possibility is to raise an Exception when an integer is given rather than a tuple/list. I think the proposed solution is more useful to the users.

¹ The Ask question uncovers two distinct problems, the second one is fixed in #28420.