Add project scaffold: gitignore, pyproject, constants, Doxyfile, CI

Closes #2
Closes #3
Closes #4
Closes #5
Closes #6
Refs #7

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Task: MIL-001#6

tests/test_constants.py

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+"""Tests for the squirrel.constants module."""
+
+import ast
+import pathlib
+import sys
+
+from squirrel import constants
+
+
+def test_fur_colors_are_gray_cinnamon_black_in_order() -> None:
+    assert constants.FUR_COLORS == ("Gray", "Cinnamon", "Black")
+
+
+def test_summary_columns_are_fur_color_and_count() -> None:
+    assert constants.SUMMARY_COLUMNS == ("Fur Color", "Count")
+
+
+def test_default_input_path_has_census_file_name() -> None:
+    assert constants.DEFAULT_INPUT_PATH.name == (
+        "2018_Central_Park_Squirrel_Census_-_Squirrel_Data.csv"
+    )
+    assert constants.DEFAULT_INPUT_PATH.parent == pathlib.Path("data")
+
+
+def test_default_output_path_has_summary_file_name() -> None:
+    assert constants.DEFAULT_OUTPUT_PATH == pathlib.Path("squirrel_count.csv")
+
+
+def test_file_encoding_and_column_name_values() -> None:
+    assert constants.FILE_ENCODING == "utf-8"
+    assert constants.FUR_COLOR_COLUMN == "Primary Fur Color"
+
+
+def test_module_imports_only_standard_library() -> None:
+    source = pathlib.Path(constants.__file__).read_text(encoding="utf-8")
+    imported: set[str] = set()
+    for node in ast.walk(ast.parse(source)):
+        if isinstance(node, ast.Import):
+            imported.update(alias.name.split(".")[0] for alias in node.names)
+        elif isinstance(node, ast.ImportFrom) and node.module is not None:
+            imported.add(node.module.split(".")[0])
+
+    assert imported <= sys.stdlib_module_names