Degree symbol `deg` rendered as angle function

The `deg` in a favunit symbol is parsed by sympy as the `deg` function that handle angle conversion.
```python
from physipy import rad, units
deg = units["deg"]
a = 5*deg
a.favunit = deg
print(a) # as expected : 5.0 deg
a
```
```
5.0\,\mathtt{\text{<function deg at 0x113c6e560>}}5.0<function deg at 0x113c6e560>
```