Replace flake8 linter with ruff linter/formatter

* By replacing flake8 with ruff we can now automatically fix the linter
  errors with `ruff check --fix`.

* ruff found an instance of a D410 D411 violation that was fixed

pyproject.toml

@@ -86,3 +86,32 @@ indent = 4
 include_trailing_comma = true
 lines_after_imports = 2
 known_first_party = "skmatter"
+
+[tool.ruff]
+exclude = ["docs/src/examples/"]
+lint.ignore = [
+    "F401",
+    "E203",
+    "D100",
+    "D101",
+    "D102",
+    "D205",
+    "D400",
+    "D401",
+]
+line-length = 88
+lint.select = [
+    "D",
+    "E",
+    "F",
+    "W",
+]
+
+[tool.ruff.lint.pydocstyle]
+convention = "numpy"
+
+[tool.ruff.lint.per-file-ignores]
+"examples/**" = [
+    "D205",
+    "D400",
+]

src/skmatter/decomposition/_kernel_pcovr.py

@@ -465,6 +465,7 @@ def score(self, X, Y):
             independent (predictor) variable
         Y : numpy.ndarray
             dependent (response) variable
+
         Returns
         -------
         L : float

tox.ini

@@ -55,14 +55,11 @@ skip_install = true
 deps =
     black
     blackdoc
-    flake8
-    flake8-bugbear
-    flake8-docstrings
-    flake8-sphinx-links
+    ruff
     isort
     sphinx-lint
 commands =
-    flake8 {[tox]lint_folders}
+    ruff check {[tox]lint_folders}
     black --check --diff {[tox]lint_folders}
     blackdoc --check --diff {[tox]lint_folders}
     isort --check-only --diff {[tox]lint_folders}
@@ -75,10 +72,12 @@ commands =
 # formatting on all files
 skip_install = true
 deps =
+    ruff
     black
     blackdoc
     isort
 commands =
+    ruff check --fix {[tox]lint_folders}
     black {[tox]lint_folders}
     blackdoc {[tox]lint_folders}
     isort {[tox]lint_folders}
@@ -92,21 +91,3 @@ extras = examples
 commands =
     sphinx-build {posargs:-E} -W -b doctest docs/src docs/build/doctest
     sphinx-build {posargs:-E} -W -b html docs/src docs/build/html
-
-[flake8]
-max_line_length = 88
-exclude =
-    docs/src/examples/
-docstring-convention = numpy
-per-file-ignores =
-    # D205 and D400 are incompatible with the requirements of sphinx-gallery
-    examples/**:D205, D400
-ignore = 
-    E203
-    D100
-    D101
-    D102
-    D205
-    D400
-    D401
-    W503