Upgrade python version to 3.11

As per discussion in DistributedProofreaders#57, this is a compromise between using 3.12
and not causing undue hassle for Linux users.

Also, while here, update black version to 23.12, which is the last
release using the 2023 stable style.

Fixes DistributedProofreaders#57

pyproject.toml

@@ -7,11 +7,11 @@ readme = "README.md"
 packages = [{include = "guiguts", from = "src"}]
 
 [tool.poetry.dependencies]
-python = "^3.10"
+python = "^3.11"
 Pillow = "^10.1"
 
 [tool.poetry.group.dev.dependencies]
-black = "^23.11"
+black = "^23.12"
 flake8 = "^6.1"
 Sphinx = "^7.2"
 pytest = "^7.4"