Pin pytest/pytest-asyncio

pytest-asyncio 0.22, 0.23 and 0.24 are broken in various ways, cf.
* pytest-dev/pytest-asyncio#670
* pytest-dev/pytest-asyncio#718
* pytest-dev/pytest-asyncio#587

Additionally, pytest 8 cannot work with pytest-asyncio 0.21, so we need
to pin pytest to 7.x too.

Perhaps there is a way to make this work, but pin it to earlier versions
to do this deliberately at some point.

pyproject.toml

@@ -41,9 +41,9 @@ dependencies = [
 ]
 optional-dependencies.tests = [
     "coverage[toml]",
-    "pytest",
+    "pytest==7.*",
     "pytest-cov",
-    "pytest-asyncio",
+    "pytest-asyncio==0.21.0",
     "irc",
     "pyyaml",
 ]