fix: use full path and os.path to version.py in setup.py (#97)

setup.py

@@ -22,10 +22,6 @@
 
 name = "google-cloud-datastore"
 description = "Google Cloud Datastore API client library"
-version = {}
-with open("google/cloud/datastore/version.py") as fp:
-    exec(fp.read(), version)
-version = version["__version__"]
 
 # Should be one of:
 # 'Development Status :: 3 - Alpha'
@@ -43,6 +39,11 @@
 
 package_root = os.path.abspath(os.path.dirname(__file__))
 
+version = {}
+with open(os.path.join(package_root, "google/cloud/datastore/version.py")) as fp:
+    exec(fp.read(), version)
+version = version["__version__"]
+
 readme_filename = os.path.join(package_root, "README.rst")
 with io.open(readme_filename, encoding="utf-8") as readme_file:
     readme = readme_file.read()