Add isort pre-commit hook, sort imports in src and test directories

tests/css/test_help_text.py

@@ -2,17 +2,17 @@ import pytest
 
 from tests.utilities.render import render
 from textual.css._help_text import (
-    spacing_wrong_number_of_values_help_text,
-    spacing_invalid_value_help_text,
-    scalar_help_text,
-    string_enum_help_text,
-    color_property_help_text,
-    border_property_help_text,
-    layout_property_help_text,
-    fractional_property_help_text,
-    offset_property_help_text,
     align_help_text,
+    border_property_help_text,
+    color_property_help_text,
+    fractional_property_help_text,
+    layout_property_help_text,
+    offset_property_help_text,
     offset_single_axis_help_text,
+    scalar_help_text,
+    spacing_invalid_value_help_text,
+    spacing_wrong_number_of_values_help_text,
+    string_enum_help_text,
     style_flags_property_help_text,
 )
 

tests/css/test_parse.py

@@ -8,7 +8,7 @@ from textual.css.parse import substitute_references
 from textual.css.scalar import Scalar, Unit
 from textual.css.stylesheet import Stylesheet, StylesheetParseError
 from textual.css.tokenize import tokenize
-from textual.css.tokenizer import Token, ReferencedBy
+from textual.css.tokenizer import ReferencedBy, Token
 from textual.css.transition import Transition
 from textual.geometry import Spacing
 from textual.layouts.vertical import VerticalLayout

tests/css/test_styles.py

@@ -1,13 +1,12 @@
 from decimal import Decimal
 
 import pytest
-
 from rich.style import Style
 
 from textual.color import Color
 from textual.css.errors import StyleValueError
 from textual.css.scalar import Scalar, Unit
-from textual.css.styles import Styles, RenderStyles
+from textual.css.styles import RenderStyles, Styles
 from textual.dom import DOMNode
 from textual.widget import Widget