Performance optimization: check for python float before type-checking numpy.

Checking whether the object is a numpy array checking is pretty expensive, so we only want to do it when the object is not trivially a float.

This makes appending to a repeated field of float ~3x faster in a common case:

```
_bench_append, 1000000, 284.1
_bench_extend, 1000000, 209.0
_bench_assign, 1000000, 175.8
_bench_pybind11, 1000000, 3.7
```

```
_bench_append, 1000000, 128.4
_bench_extend, 1000000, 67.1
_bench_assign, 1000000, 57.2
_bench_pybind11, 1000000, 3.5
```

PiperOrigin-RevId: 707811151