Fix px_uploader.py to work in 16.04 with Python 2.7.

Before, the pyserial check would fail before it could check for the
VERSION that the Python 2.7 version contains. This fixes it to check
for the VERSION independently.

Tools/px_uploader.py

@@ -727,10 +727,20 @@ def main():
 
     # We need to check for pyserial because the import itself doesn't
     # seem to fail, at least not on macOS.
+    pyserial_installed = False
     try:
-        if serial.__version__ or serial.VERSION:
-            pass
+        if serial.__version__:
+            pyserial_installed = True
     except:
+        pass
+    
+    try:
+        if serial.VERSION:
+            pyserial_installed = True
+    except:
+        pass
+    
+    if not pyserial_installed:
         print("Error: pyserial not installed!")
         print("    (Install using: sudo pip install pyserial)")
         sys.exit(1)