fix: harden install to use full paths, and windows separators on windows (#88)

Author: crwilcox

setup.py

@@ -23,11 +23,6 @@
 name = "google-api-core"
 description = "Google API client core library"
 
-version = {}
-with open("google/api_core/version.py") as fp:
-    exec(fp.read(), version)
-version = version['__version__']
-
 # Should be one of:
 # 'Development Status :: 3 - Alpha'
 # 'Development Status :: 4 - Beta'
@@ -54,6 +49,12 @@
 
 package_root = os.path.abspath(os.path.dirname(__file__))
 
+
+version = {}
+with open(os.path.join(package_root, "google/api_core/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()