fix: Pin setuptools to avoid distutils error

The newest version of setuptools (65.6.x) breaks httpstan's code---that
has been working for years. This commit adds a version pin that should
be removed once the issue is resolved.

pyproject.toml

@@ -31,7 +31,7 @@ include = [
 
 [tool.poetry.dependencies]
 python = "^3.8"
-setuptools = ">=48.0"
+setuptools = ">=48.0,<65.6"  # NOTE: <65.6 works around a distutils issue, remove pin soon (see PR#627)
 aiohttp = "^3.7"
 appdirs = "^1.4"
 webargs = "^8.0"