Add lint of python files in CI

[nattvara]

Description

Add linting of python files in the CI

Motivation and Context

Ensure the files are neat and tidy.

What will this PR change

• Add a lint step in the CI
• Fix the lint errors

Notes

The lint issues

./db/custom_fields/__init__.py:1:1: F401 '.arrow_datetime_field.ArrowDateTimeField' imported but unused
./db/custom_fields/__init__.py:2:1: F401 '.model_params.ModelParamsField' imported but unused
./db/custom_fields/__init__.py:3:1: F401 '.model_name.ModelNameField' imported but unused
./db/migrations/001_add_prompt_handles_table.py:9:5: F401 'playhouse.postgres_ext as pw_pext' imported but unused
./db/models/__init__.py:1:1: F401 '.base_model.BaseModel' imported but unused
./db/models/prompt_handle.py:11:121: E501 line too long (158 > 120 characters)
./http_api/websocket_protocols/llm_streams.py:97:121: E501 line too long (121 > 120 characters)
./llms/generate.py:14:121: E501 line too long (134 > 120 characters)
./llms/generate.py:93:121: E501 line too long (129 > 120 characters)
./llms/sampling_strategies.py:6:121: E501 line too long (129 > 120 characters)
./llms/sampling_strategies.py:7:121: E501 line too long (129 > 120 characters)
./services/llm/worker.py:58:29: F541 f-string is missing placeholders
./services/llm/worker.py:79:27: F541 f-string is missing placeholders
./services/llm/worker.py:81:21: F541 f-string is missing placeholders
./tests/conftest.py:16:1: E402 module level import not at top of file
./tests/conftest.py:17:1: E402 module level import not at top of file
./tests/conftest.py:18:1: E402 module level import not at top of file
./tests/conftest.py:19:1: E402 module level import not at top of file
./tests/conftest.py:80:121: E501 line too long (129 > 120 characters)
./tests/services/llm/llm_test.py:140:52: E712 comparison to False should be 'if cond is False:' or 'if not cond:'
./tests/services/llm/llm_test.py:142:52: E712 comparison to False should be 'if cond is False:' or 'if not cond:'
./tests/services/llm/llm_test.py:148:1: F811 redefinition of unused 'test_model_params_can_be_specified' from line 119
./tests/services/llm/worker_test.py:32:121: E501 line too long (133 > 120 characters)
./tests/services/llm/worker_test.py:40:121: E501 line too long (122 > 120 characters)
./tests/services/llm/worker_test.py:89:121: E501 line too long (136 > 120 characters)
./tests/services/llm/worker_test.py:128:121: E501 line too long (139 > 120 characters)
./tests/services/llm/worker_test.py:145:121: E501 line too long (125 > 120 characters)
4     E402 module level import not at top of file
12    E501 line too long (158 > 120 characters)
2     E712 comparison to False should be 'if cond is False:' or 'if not cond:'
5     F401 '.arrow_datetime_field.ArrowDateTimeField' imported but unused
3     F541 f-string is missing placeholders
1     F811 redefinition of unused 'test_model_params_can_be_specified' from line 119
27