Hopefully definite fix for building from outside package directory (follow-up for 73ec7da13e)

`find_packages` does not seem to work correctly when the working directory does not equal the package directory (resulted in the actual module not being installed).
Fix this by changing the working directory during setup.

setup.py

@@ -32,7 +32,11 @@ Authors:
   - Tobias Oberstein (maintainer)
 """
 
-VERSIONFILE = os.path.join(os.path.dirname(os.path.realpath(__file__)), "scour", "__init__.py")
+old_path = os.getcwd()
+src_path = os.path.dirname(os.path.realpath(__file__))
+os.chdir(src_path)
+
+VERSIONFILE = "scour/__init__.py"
 verstrline = open(VERSIONFILE, "rt").read()
 VSRE = r"^__version__ = u['\"]([^'\"]*)['\"]"
 mo = re.search(VSRE, verstrline, re.M)
@@ -74,3 +78,5 @@ setup (
                   "Topic :: Utilities"],
    keywords = 'svg optimizer'
 )
+
+os.chdir(old_path)