wpt: Make WPT result formatting logic independent of ServoHandler

This will allow results to be formatted by other parts of the code (such
as the intermittent filtering) code. Previously, formatting was handled
in ServoHandler, which was a bit strange as it's really only necessary
for GroupingFormatter and the intermittent filtering code. This also
allows the results to be properly typed by the Python typing system.

python/requirements.txt

@@ -8,6 +8,7 @@ mozinfo == 1.2.1
 mozlog == 7.1.0
 setuptools == 65.5.1
 toml == 0.9.2
+dataclasses == 0.8; python_version < "3.7"
 
 # For Python linting
 flake8 == 3.8.3