"ValueError: substring not found" in textbox draw

Nobody has claimed this yet.

Hey,
I was recently digging around the text fitting source (ptext.py) and found an issue.

When calling `screen.draw.textbox`, if you prefix the string with a space(s), it will cause a `ValueError` exception to be thrown.

Without the space at the beginning, it works just fine.

Thank you for your time.

### Source Code
```python
import pgzrun

WIDTH = 500
HEIGHT = 500

text = " ab"

def draw():
screen.draw.textbox(text, (0, 0, 500, 500))

pgzrun.go()
```
### Stack Trace
```
File "C:\..\issue.py", line 9, in draw
screen.draw.textbox(text, (0, 0, 500, 500))
File "C:\..\lib\site-packages\pgzero\screen.py", line 66, in textbox
ptext.drawbox(*args, surf=self._surf, **kwargs)
File "C:\..\lib\site-packages\pgzero\ptext.py", line 476, in drawbox
fontsize = _fitsize(text, fontname, sysfontname, bold, italic, underline,
File "C:\..\lib\site-packages\pgzero\ptext.py", line 162, in _fitsize
if not fits(a):
File "C:\..\lib\site-packages\pgzero\ptext.py", line 153, in fits
texts = wrap(text, fontname, fontsize, sysfontname,
File "C:\..\lib\site-packages\pgzero\ptext.py", line 109, in wrap
a = text.index(" ", a) if " " in text else len(text)
ValueError: substring not found
```

### System Information
>Pygame 2.0.1
SDL 2.0.14, Python 3.9.2
Windows 10.0.19041.867

Start in ptext.py, especially wrap around the reported line 109 and the _fitsize/drawbox call path. Reproduce the failure with the supplied textbox example using text=" ab"; done means the leading-space input no longer raises ValueError and the textbox draws successfully.

Written by the indexing model from the issue text.