Drop exceptiongroup dependency in Python 3.11 and later

All of the imports are already conditionalized on the Python interpreter
version; there is no need to install this backport package on current
interpreters.

pyproject.toml

@@ -28,7 +28,7 @@ documentation = "https://hypercorn.readthedocs.io"
 [tool.poetry.dependencies]
 python = ">=3.8"
 aioquic = { version = ">= 0.9.0, < 1.0", optional = true }
-exceptiongroup = ">= 1.1.0"
+exceptiongroup = { version = ">= 1.1.0", python = "<3.11" }
 h11 = "*"
 h2 = ">=3.1.0"
 priority = "*"