Move conftest.py to the python package root (#252)

This is mostly to make it easier to navigate to the root of the package.

We also fix its location in `MANIFEST.in` to ensure it is excluded from
the binary distribution.

Author: llucax

MANIFEST.in

@@ -6,7 +6,7 @@ exclude CODEOWNERS
 exclude CONTRIBUTING.md
 exclude mkdocs.yml
 exclude noxfile.py
-exclude src/conftest.py
+exclude py/frequenz/api/common/conftest.py
 recursive-exclude .github *
 recursive-exclude benchmarks *
 recursive-exclude docs *