Chore: Format code using Ruff, and fix linter errors (#531)

## About
- Add Ruff configuration to `pyproject.toml`, apply its formatter, and
satisfy its linter.
- Migrate pytest configuration to `pyproject.toml`.

pyproject.toml

@@ -1,3 +1,70 @@
 [build-system]
-requires = ["setuptools", "wheel"]
-build-backend = "setuptools.build_meta:__legacy__"
+build-backend = "setuptools.build_meta"
+requires = [
+  "setuptools>=42", # At least v42 of setuptools required.
+]
+
+[tool.ruff]
+line-length = 90
+
+extend-exclude = [
+  "docs/source/conf.py",
+  "setup.py",
+]
+
+lint.select = [
+  # Builtins
+  "A",
+  # Bugbear
+  "B",
+  # comprehensions
+  "C4",
+  # Pycodestyle
+  "E",
+  # eradicate
+  "ERA",
+  # Pyflakes
+  "F",
+  # isort
+  "I",
+  # pandas-vet
+  "PD",
+  # return
+  "RET",
+  # Bandit
+  "S",
+  # print
+  "T20",
+  "W",
+  # flake8-2020
+  "YTT",
+]
+
+lint.extend-ignore = [
+  "S101", #  Allow use of `assert`.
+]
+
+lint.per-file-ignores."tests/*" = [
+  "ERA001", # Found commented-out code.
+  "S101",   # Allow use of `assert`, and `print`.
+]
+
+[tool.pytest.ini_options]
+addopts = """
+  -rfEXs -p pytester --strict-markers --verbosity=3
+  --cov --cov-report=term-missing --cov-report=xml
+  """
+filterwarnings = [
+  "error::UserWarning",
+]
+log_level = "DEBUG"
+log_cli_level = "DEBUG"
+log_format = "%(asctime)-15s [%(name)-36s] %(levelname)-8s: %(message)s"
+minversion = "2.0"
+testpaths = [
+  "responder",
+  "tests",
+]
+markers = [
+]
+xfail_strict = true