bitmap property for bitmap_label

[FoamyGuy]

Formerly it was possible to access the Bitmap object that is drawn into by BitmapLabel. The Dial widget makes use of this in order to apply rotation to labels using bitmaptools.rotozoom()

This PR adds a bitmap property for BitmapLabel that restores access to it.

[lesamouraipourpre]

Looks good to me. Tested on PyPortal.

Could you make the following changes to documentation:

diff --git a/adafruit_display_text/bitmap_label.py b/adafruit_display_text/bitmap_label.py
index 2ef4a72..0944b31 100755
--- a/adafruit_display_text/bitmap_label.py
+++ b/adafruit_display_text/bitmap_label.py
@@ -39,8 +39,9 @@ from adafruit_display_text import LabelBase
 
 class Label(LabelBase):
     """A label displaying a string of text that is stored in a bitmap.
-    Note: This ``bitmap_label.py`` library utilizes a bitmap to display the text.
-    This method is memory-conserving relative to ``label.py``.
+
+    Note: The `bitmap_label.Label` class utilizes a :py:class:`~displayio.Bitmap` to
+    display the text. This is memory-conserving relative to `label.Label`.
 
     For further reduction in memory usage, set ``save_text=False`` (text string will not
     be stored and ``line_spacing`` and ``font`` are immutable with ``save_text``
@@ -537,5 +538,9 @@ class Label(LabelBase):
 
     @property
     def bitmap(self):
-        """The Bitmap object that the text and background are drawn into."""
+        """
+        The Bitmap object that the text and background are drawn into.
+
+        :rtype: displayio.Bitmap
+        """
         return self._bitmap

[FoamyGuy]

@lesamouraipourpre thanks for taking a look and trying this out.

Latest commit adds those type links in the docstrings.

[lesamouraipourpre]

LGTM