Extend rfc3433.py to support more Physical Entity Sensor MIB entries

**- What I did**

Extend RFC3433 implementation with:

1. FAN tachometers
2. PSU current sensor
3. PSU voltage sensor
4. PSU power sensor
5. PSU temp sensor
6. Chassis temp sensor

MIB HLD update PR to reflect this change please refer to: sonic-net/SONiC#766

A fix for the LGTM checker

**- How I did it**
1. Refactor sensor data parsing class by adding a base class  BaseSensorData;  inherit  TransceiverSensorData, PSUSensorData, FANSensorData, and ThermalSensorData from it to reduce redundant code.
2. Adding more sensor MIB entry class: PSUTempSensor, PSUVoltageSensor, PSUCurrentSensor, PSUPowerSensor, FANSpeedSensor, and ThermalSensor.
3. Separate MIB update to different functions according to different sensors types: update_xcvr_dom_data, update_psu_sensor_data, update_fan_sensor_data, and update_thermal_sensor_data.
4. Add unit test cases to cover the new added MIB entries.
5. Add lgtm.yaml to fix the LGTM checker.

**- How to verify it**

Manual test and run updated community SNMP test case(sonic-net/sonic-mgmt#3357).

lgtm.yml

@@ -0,0 +1,25 @@
+extraction:
+  python:
+    python_setup:
+      version: "3"
+    prepare:
+      packages:
+      - libxml-simple-perl
+      - aspell
+      - aspell-en
+      - libhiredis-dev
+      - libnl-3-dev
+      - libnl-genl-3-dev
+      - libnl-route-3-dev
+      - libnl-nf-3-dev
+      - libzmq3-dev
+      - libzmq5
+      - swig3.0
+      - libpython2.7-dev
+      - libgtest-dev
+      - dh-exec
+      - doxygen
+      - graphviz
+    after_prepare:
+      - git clone https://github.com/Azure/sonic-py-swsssdk.git; pushd sonic-py-swsssdk; python3 setup.py bdist_wheel; popd
+      - pip3 install --upgrade --user sonic-py-swsssdk/dist/swsssdk-2.0.1-py3-none-any.whl

src/sonic_ax_impl/mibs/__init__.py

@@ -25,6 +25,8 @@
 TABLE_NAME_SEPARATOR_COLON = ':'
 TABLE_NAME_SEPARATOR_VBAR = '|'
 
+HOST_NAMESPACE_DB_IDX = 0
+
 redis_kwargs = {'unix_socket_path': '/var/run/redis/redis.sock'}
 
 

src/sonic_ax_impl/mibs/ietf/rfc2737.py

@@ -21,8 +21,7 @@
 from .physical_entity_sub_oid_generator import get_psu_sensor_sub_id
 from .physical_entity_sub_oid_generator import get_transceiver_sub_id
 from .physical_entity_sub_oid_generator import get_transceiver_sensor_sub_id
-from .transceiver_sensor_data import TransceiverSensorData
-
+from .sensor_data import TransceiverSensorData
 
 @unique
 class PhysicalClass(int, Enum):