doc/api/signaltypes: use ^{\circ} instead of ° in :math:

This works for both sphinx.ext.mathjax and sphinx.ext.imgmath.

doc/api/signaltypes.rst

@@ -248,9 +248,9 @@ temperature: °C
 Temperature is measured in degrees Celcius (°C). To convert to degrees
 Fahrenheit (°F) or Kelvin (K), you can use the following conversion formulas:
 
-    :math:`°\kern1pt\!F = °\kern2pt\!C \cdot \frac{9}{5} + 32`.
+    :math:`^{\circ}\kern1pt\!F =\kern1pt^{\circ}\kern1pt\!C \cdot \frac{9}{5} + 32`.
 
-    :math:`K = °\kern2pt\!C + 273.15`.
+    :math:`K =\kern1pt^{\circ}\kern1pt\!C + 273.15`.
 
 .. _hue:
 

setup.cfg

@@ -5,3 +5,4 @@ max-line-length = 88
 
 [doc8]
 ignore-path = .venv/,doc/api/build/,pybricks.egg-info/
+max-line-length = 88