[enable_counters.py] Convert to Python 3 (#5789)

**- Why I did it**

As part of moving all SONiC code from Python 2 (no longer supported) to Python 3

**- How I did it**

- Convert enable_counters.py script to Python 3
- Reorganize imports per PEP8 standard
- Two blank lines precede functions per PEP8 standard

dockers/docker-orchagent/enable_counters.py

@@ -1,13 +1,16 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
-import swsssdk
 import time
 
+import swsssdk
+
+
 def enable_counter_group(db, name):
     info = {}
     info['FLEX_COUNTER_STATUS'] = 'enable'
     db.mod_entry("FLEX_COUNTER_TABLE", name, info)
 
+
 def enable_counters():
     db = swsssdk.ConfigDBConnector()
     db.connect()
@@ -20,10 +23,12 @@ def enable_counters():
     enable_counter_group(db, 'BUFFER_POOL_WATERMARK')
     enable_counter_group(db, 'PORT_BUFFER_DROP')
 
+
 def get_uptime():
     with open('/proc/uptime') as fp:
         return float(fp.read().split(' ')[0])
 
+
 def main():
     # If the switch was just started (uptime less than 5 minutes),
     # wait for 3 minutes and enable counters
@@ -35,5 +40,6 @@ def main():
         time.sleep(60)
     enable_counters()
 
+
 if __name__ == '__main__':
     main()