Add support for Python 3 in //devtools/cdbg/e2e/python/gce:e2e_python_gce_test

Unfortunately tools like virtualenv and pyenv won't work with this since they
are designed for interactive shells and _vm.RunShellCommand starts a new one
every time.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187532256

Author: xinghuadou-google

src/build.sh

@@ -84,6 +84,7 @@ library_dirs=${ROOT}/build/third_party/lib" > ${ROOT}/setup.cfg
 
 # Build the Python Cloud Debugger agent.
 pushd ${ROOT}
-python setup.py bdist_egg
+# Use custom python command if variable is set
+"${PYTHON:-python}" setup.py bdist_egg
 popd
 

src/googleclouddebugger/__init__.py

@@ -143,7 +143,9 @@ def _DebuggerMain():
 
   sys.modules['__main__'] = __main__
 
-  exec('execfile(%r)' % app_path, globals, locals)  # pylint: disable=exec-used
+  with open(app_path) as f:
+    code = compile(f.read(), app_path, 'exec')
+    exec(code, globals, locals)  # pylint: disable=exec-used
 
 
 # pylint: disable=invalid-name

src/googleclouddebugger/gcp_hub_client.py

@@ -461,6 +461,8 @@ def _QueryGcpProject(self, resource):
           'HTTP error %s %s when querying local metadata service at %s' %
           (response['status'], content, url))
 
+    if not isinstance(content, str):
+      content = content.decode()
     return content
 
   def _GetDebuggee(self):