Pretty printing using MIDDLE DOT unicode character instead of DOT OPERATOR

The pretty printing uses "·" U+00B7 (MIDDLE DOT) instead of the more appropriate "⋅" (U+22C5, DOT OPERATOR)

Example:

```python
from pint import UnitRegistry
ureg = UnitRegistry()
f"{ureg("kg*m").units:~P}"
'kg·m'
```

The output should be
`kg⋅m`