bump minimum version of Pillow to actual required minimum

In commit a4fc7a1 handling of
Image.Transpose enum members was added, which is introduced in Pillow
9.1.0 as the superior way to handle constants:

https://pillow.readthedocs.io/en/latest/deprecations.html#constants

Previously, the minimum version was rather low. Make sure people cannot
accidentally install WeasyPrint with too old Pillow.

docs/first_steps.rst

@@ -17,7 +17,7 @@ WeasyPrint |version| depends on:
 * tinycss2_ ≥ 1.0.0
 * cssselect2_ ≥ 0.1
 * Pyphen_ ≥ 0.9.1
-* Pillow_ ≥ 4.0.0
+* Pillow_ ≥ 9.1.0
 * fontTools_ ≥ 4.0.0
 
 .. _Python: https://www.python.org/

pyproject.toml

@@ -18,7 +18,7 @@ dependencies = [
   'tinycss2 >=1.0.0',
   'cssselect2 >=0.1',
   'Pyphen >=0.9.1',
-  'Pillow >=4.0.0',
+  'Pillow >=9.1.0',
   'fonttools[woff] >=4.0.0',
 ]
 classifiers = [