space.Integer and space.Float not allowed to be constant

[spiiph]

The Integer.constant property makes a comparison self.low == self.high, but this line prevents low and high from being set to equal values:

scikit-optimize/skopt/space/space.py

Line 436 in de32b5f

if high <= low:

This does not seem intended, and that line 436 should read:

        if high < low:

The same applies to Float.