Tools: use Python 3, improve import checks

In more detail:
- Change shebang to Python 3.
- Suggest installation using pip3 as user.

Tools/upload_log.py

@@ -16,11 +16,13 @@ import sys
 
 try:
     import requests
-except:
-    print("Failed to import requests.")
-    print("You may need to install it with 'pip install requests'")
+except ImportError as e:
+    print("Failed to import requests: " + e)
     print("")
-    raise
+    print("You may need to install it using:")
+    print("    pip3 install --user requests")
+    print("")
+    sys.exit(1)
 
 
 SERVER = 'https://logs.px4.io'