refactor(database/prometheus): Remove redundant ABC inheritance

The `Collector` abstract class from the Prometheus client already uses
`ABCMeta` as its metaclass, so having `ReportCollector` inheriting it as
well can cause a metaclass conflict on some Python versions/builds.

Author: gfieni

src/powerapi/database/prometheus/collectors.py

@@ -47,7 +47,7 @@
     Collector = None
 
 
-class ReportCollector(Collector, ABC):
+class ReportCollector(Collector):
     """
     Base Report Collector class.
     """