Fix dynamic minimum fan table issue caused by python3 (#6690)

**- Why I did it**
After migrating to python3, the operator '/' always get a float result, but it gets integer result in python2. Need fix this in thermal_conditions.

**- How I did it**
1. cast float value to int
2. change the unit test case to cover this situation

**- How to verify it**
Manually test and regression test

platform/mellanox/mlnx-platform-api/sonic_platform/thermal_conditions.py

@@ -85,7 +85,7 @@ def is_match(self, thermal_info_dict):
 
         trust_state = Thermal.check_module_temperature_trustable()
         temperature = Thermal.get_min_amb_temperature()
-        temperature = temperature / 1000
+        temperature = int(temperature / 1000)
 
         change_cooling_level = False
         if trust_state != MinCoolingLevelChangeCondition.trust_state:

platform/mellanox/mlnx-platform-api/tests/test_thermal_policy.py

@@ -482,7 +482,7 @@ def test_dynamic_minimum_policy(thermal_manager):
     condition = policy.conditions[MinCoolingLevelChangeCondition]
     action = policy.actions[ChangeMinCoolingLevelAction]
     Thermal.check_module_temperature_trustable = MagicMock(return_value='trust')
-    Thermal.get_min_amb_temperature = MagicMock(return_value=35000)
+    Thermal.get_min_amb_temperature = MagicMock(return_value=35001)
     assert condition.is_match(None)
     assert MinCoolingLevelChangeCondition.trust_state == 'trust'
     assert MinCoolingLevelChangeCondition.temperature == 35
@@ -492,7 +492,7 @@ def test_dynamic_minimum_policy(thermal_manager):
     assert condition.is_match(None)
     assert MinCoolingLevelChangeCondition.trust_state == 'untrust'
 
-    Thermal.get_min_amb_temperature = MagicMock(return_value=25000)
+    Thermal.get_min_amb_temperature = MagicMock(return_value=25999)
     assert condition.is_match(None)
     assert MinCoolingLevelChangeCondition.temperature == 25