Look for ini file to spot python project root (#755)

* Look for ini file to spot project root

When looking for the project root folder it would be better
to check for some well-known init file instead of __init__.py.
Indeed, with python3 it is now possible to have namespace modules
where intermediate dirs are not required to include the __init__.py file.

* Break if statement conditions over several lines

* Add blank lines for the if block

* Add test for FindProjectRoot

* Typo: missing / for MANIFEST.in

* Fix test for non-namespace package

* Add more test cases

test/command_callback/python_paths/namespace_package_manifest/MANIFEST.in

@@ -0,0 +1,3 @@
+include README.md
+include *.ini *.cfg *.txt
+include requirements/*.txt