Add tox for tests

Enable tox and clean up for lightweight tests.

- Enable tox, testing, formatting and linting for all supported versions of
  Python
- Clean out unused imports and replace wildcard imports with explicit
  imports

I encountered this error on running tox:

```
AttributeError: module 'pkgutil' has no attribute 'ImpImporter'. Did you
mean: 'zipimporter'?
```

The fix was to replace my tox venv with a new one built from a recent
version of Python and install it with an fresh installation of pipx.

Author: dspruell-i01

src/md_insights_client/exceptions.py

@@ -7,6 +7,3 @@ class ConfigurationError(Exception):
 
 class FeedAccessError(Exception):
     "Exception indicating a problem accessing the feed"
-
-
-__all__ = ["ConfigurationError", "FeedAccessError"]

src/md_insights_client/insights_snapshot.py

@@ -5,18 +5,22 @@
 
 import gzip
 import logging
-import os
 import re
 import shutil
-import sys
 from argparse import ArgumentParser
 from datetime import datetime
 from importlib.metadata import version
 
 import requests
 
-from .exceptions import *
-from .settings import *
+from .exceptions import ConfigurationError, FeedAccessError
+from .settings import (
+    CHOICE_LOG_LEVELS,
+    CONFIG_FILE_DEFAULT,
+    DEFAULT_LOGLEVEL,
+    MD_INSIGHTS_API_HOST,
+    SettingsLoader,
+)
 
 __application_name__ = "md-insights-client"
 __version__ = version(__application_name__)
@@ -66,9 +70,8 @@ def fetch_feeds(api_key, feeds=[]):
                 logging.error(
                     "response body from server response: %s", response.text
                 )
-            parser.exit(
-                status=1,
-                message=f"API error attempting to retrieve feed {feed}: {e}\n",
+            raise FeedAccessError(
+                f"API error attempting to retrieve feed: {e}"
             )
 
         # Extract filename from response header

src/md_insights_client/settings.py

@@ -51,12 +51,3 @@ def __init__(self, conf_file: str = CONFIG_FILE_DEFAULT):
 
     def get_config(self):
         return self.config
-
-
-__all__ = [
-    "CHOICE_LOG_LEVELS",
-    "CONFIG_FILE_DEFAULT",
-    "DEFAULT_LOGLEVEL",
-    "MD_INSIGHTS_API_HOST",
-    "SettingsLoader",
-]

tests/.keep

@@ -0,0 +1,6 @@
+"""Tests."""
+
+
+def test_always_passes():
+    """Test that always passes."""
+    assert True

tests/test_nop.py

@@ -0,0 +1,28 @@
+[tox]
+requires =
+    tox
+env_list = format, lint, py{39,310,311,312,313}
+
+[testenv]
+description = run unit tests
+deps =
+    pytest
+    pytest-sugar
+commands =
+    pytest {posargs:tests}
+
+[testenv:format]
+description = run formatters
+skip_install = true
+deps =
+    black
+commands =
+    black -l 79 {posargs:src}
+
+[testenv:lint]
+description = run linters
+skip_install = true
+deps =
+    flake8
+commands =
+    flake8 {posargs:src}