Tools: use Python 3, improve import checks

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

Tools/mavlink_ulog_streaming.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 Stream ULog data over MAVLink.
@@ -12,15 +12,17 @@ import sys, select, os
 import datetime
 from timeit import default_timer as timer
 os.environ['MAVLINK20'] = '1' # The commands require mavlink 2
+from argparse import ArgumentParser
 
 try:
     from pymavlink import mavutil
-except:
-    print("Failed to import pymavlink.")
-    print("You may need to install it with 'pip install pymavlink pyserial'")
+except ImportError as e:
+    print("Failed to import pymavlink: " + e)
     print("")
-    raise
-from argparse import ArgumentParser
+    print("You may need to install it with:")
+    print("    pip3 install --user pymavlink")
+    print("")
+    sys.exit(1)
 
 
 class MavlinkLogStreaming():