Move css style related to scwidgets.css_style level

The css style related functions are not essential for creating a
exercise notebook so we move it one level higher. For that we rename the
`_css_style.py` to `css_style.py`.

src/scwidgets/__init__.py

@@ -1,16 +1,12 @@
 __version__ = "0.1.0-dev0"
 __authors__ = "the scicode-widgets developer team"
 
-from ._css_style import CssStyle, get_css_style
 from .check import *  # noqa: F403
 from .code import *  # noqa: F403
 from .cue import *  # noqa: F403
 from .exercise import *  # noqa: F403
 
 __all__ = [  # noqa: F405
-    # css_style
-    "CssStyle",
-    "get_css_style",
     # cue
     "CueWidget",
     "CheckCueBox",

src/scwidgets/check/_widget_check_registry.py

@@ -7,7 +7,7 @@
 
 from ipywidgets import Button, HBox, Layout, Output, VBox, Widget
 
-from .._css_style import CssStyle
+from ..css_style import CssStyle
 from .._utils import Formatter
 from ._check import Check, CheckResult
 

src/scwidgets/exercise/_widget_code_exercise.py

@@ -12,7 +12,7 @@
 from widget_code_input import WidgetCodeInput
 from widget_code_input.utils import CodeValidationError
 
-from .._css_style import CssStyle
+from ..css_style import CssStyle
 from .._utils import Formatter
 from ..check import Check, CheckableWidget, CheckRegistry, CheckResult
 from ..code._widget_code_input import CodeInput

src/scwidgets/exercise/_widget_exercise_registry.py

@@ -11,7 +11,7 @@
 from IPython.display import display
 from ipywidgets import Button, Dropdown, HBox, Label, Layout, Output, Text, VBox
 
-from .._css_style import CssStyle
+from ..css_style import CssStyle
 from .._utils import Formatter
 
 

src/scwidgets/exercise/_widget_text_exercise.py

@@ -2,7 +2,7 @@
 
 from ipywidgets import HTML, HBox, HTMLMath, Layout, Output, Textarea, VBox
 
-from .._css_style import CssStyle
+from ..css_style import CssStyle
 from .._utils import Formatter
 from ..cue import SaveCueBox, SaveResetCueButton
 from ._widget_exercise_registry import ExerciseRegistry, ExerciseWidget