BUG/CLN: Clean float / complex string formatting
Noticed while working on another bug. The _is_number helper here is wrong and can cause incorrect results given that this code path is hit by arbitrary strings (e.g., it thinks "foo" is a number). Also the _trim_zeros_complex helper apparently does nothing:
```python
[ins] In [3]: _trim_zeros_float(["0.00000"])
Out[3]: ['0.0']

[ins] In [4]: _trim_zeros_complex(["1.000+1.000000j"])
Out[4]: ['1.000+1.000000j']
```