pythonPackages.trio: fix build (#46342)

Fixes the build for `python3Packages.trio' for the next ZHF iteration.
Please refer to the Hydra build for further reference: https://hydra.nixos.org/build/80617356

`python3Packages.sniffio` is needed for the build, otherwise the build
aborts with an error like this:

```
  Could not find a version that satisfies the requirement sniffio (from trio==0.6.0) (from versions: )
No matching distribution found for sniffio (from trio==0.6.0)
```

See #45960

pkgs/development/python-modules/sniffio/default.nix

@@ -0,0 +1,30 @@
+{ buildPythonPackage, lib, fetchPypi, glibcLocales, isPy3k, contextvars
+, pythonOlder
+}:
+
+buildPythonPackage rec {
+  pname = "sniffio";
+  version = "1.0.0";
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "1dzb0nx3m1hpjgsv6s6w5ac2jcmywcz6gqnfkw8rwz1vkr1836rf";
+  };
+
+  # breaks with the following error:
+  # > TypeError: 'encoding' is an invalid keyword argument for this function
+  disabled = !isPy3k;
+
+  buildInputs = [ glibcLocales ];
+
+  propagatedBuildInputs = lib.optionals (pythonOlder "3.7") [ contextvars ];
+
+  # no tests distributed with PyPI
+  doCheck = false;
+
+  meta = with lib; {
+    homepage = https://github.com/python-trio/sniffio;
+    license = licenses.asl20;
+    description = "Sniff out which async library your code is running under";
+  };
+}