Fix validation of numpy types depending on numpy being imported.

If numpy was not imported before traits, validators did not consider
numpy types.

E.g.

```
import numpy
from traits.api import Float, HasTraits

class A(HasTraits):
    n = Float

a = A()
a.n = numpy.float32(0.5)  # suceeds
```

```
from traits.api import Float, HasTraits
import numpy

class A(HasTraits):
    n = Float

a = A()
a.n = numpy.float32(0.5)  # fails
```