in conf2reST, use pip3 when pip is not found

conf2reST.py

@@ -97,7 +97,15 @@
 
     # pip freeze
     reqs = dict()
-    for element in subprocess.check_output(['pip', 'freeze']).split():
+    try:
+        pip_freeze = subprocess.check_output(['pip', 'freeze'])
+    except OSError as e:
+        if e.errno == os.errno.ENOENT:
+            pip_freeze = subprocess.check_output(['pip3', 'freeze'])
+        else:
+            # Something else went wrong
+            raise
+    for element in pip_freeze.split():
         lib_and_version = element.decode('utf-8').split('==')
         if len(lib_and_version) == 1:  # No explicit version (no ==).
             library, version = lib_and_version[0], "N/A"