feat: create libretime_shared package (#1349)

* feat: create libretime_shared package

- We don't use pydantic.BaseSettings because of some
  incompatble loading behavior.

* fix: whitelist pydantic in pylintrc

* docs: update to new BaseConfig behavior

* docs: change confusing config filepath

shared/tests/config_test.py

@@ -0,0 +1,72 @@
+from os import environ
+from unittest import mock
+
+from pytest import mark, raises
+
+from libretime_shared.config import BaseConfig, Database
+
+
+# pylint: disable=too-few-public-methods
+class FixtureConfig(BaseConfig):
+    api_key: str
+    database: Database
+
+
+FIXTURE_CONFIG_RAW = """
+api_key: "f3bf04fc"
+
+# Comment !
+database:
+    host: "localhost"
+    port: 5672
+
+ignored: "ignored"
+"""
+
+
+def test_base_config(tmpdir):
+    config_filepath = tmpdir.join("config.yml")
+    config_filepath.write(FIXTURE_CONFIG_RAW)
+
+    with mock.patch.dict(
+        environ,
+        dict(
+            LIBRETIME_API="invalid",
+            LIBRETIME_API_KEY="f3bf04fc",
+            LIBRETIME_DATABASE="invalid",
+            LIBRETIME_DATABASE_PORT="8888",
+            WRONGPREFIX_API_KEY="invalid",
+        ),
+    ):
+        config = FixtureConfig(filepath=config_filepath)
+
+        assert config.api_key == "f3bf04fc"
+        assert config.database.host == "localhost"
+        assert config.database.port == 8888
+
+
+FIXTURE_CONFIG_RAW_MISSING = """
+database:
+    host: "localhost"
+"""
+
+FIXTURE_CONFIG_RAW_INVALID = """
+database
+    host: "localhost"
+"""
+
+
+@mark.parametrize(
+    "raw,exception",
+    [
+        (FIXTURE_CONFIG_RAW_INVALID, SystemExit),
+        (FIXTURE_CONFIG_RAW_MISSING, SystemExit),
+    ],
+)
+def test_load_config_error(tmpdir, raw, exception):
+    config_filepath = tmpdir.join("config.yml")
+    config_filepath.write(raw)
+
+    with raises(exception):
+        with mock.patch.dict(environ, {}):
+            FixtureConfig(filepath=config_filepath)