Pin pytest-asyncio to version 0.21.1

Prevents issue reported here and revert is the suggested way:
pytest-dev/pytest-asyncio#706

Tests in this repository are not affected at the moment because event
loops are used only in function scope fixtures but this prevents
possible confusion in the future when that would not be the case.

flake.nix

@@ -16,7 +16,9 @@
     with nixpkgs.lib; let
       pyproject = trivial.importTOML ./pyproject.toml;
 
-      pypy2nix_map = {};
+      pypy2nix_map = {
+        "pytest-asyncio" = "pytest-asyncio_0_21";
+      };
       list2attr = list: attr: attrValues (getAttrs list attr);
       pypi2nix = list:
         list2attr (map (n: let

pyproject.toml

@@ -33,7 +33,7 @@ dependencies = [
 [project.optional-dependencies]
 test = [
   "pytest",
-  "pytest-asyncio",
+  "pytest-asyncio == 0.21.1",
   "pytest-cov",
 ]
 docs = [