Revert "Merge #43190: libyaml: 0.1.7 -> 0.2.1"

This reverts commit 0f57d85, reversing
changes made to 65af430.

I'm missing human time to solve errors or verify that they're harmless...

pkgs/development/libraries/libyaml/default.nix

@@ -1,13 +1,14 @@
 { stdenv, fetchurl }:
 let
-  version = "0.2.1";
+  # 0.2.1 broke the tests of pythonPackages.pyyaml 3.13
+  version = "0.1.7";
 in
 stdenv.mkDerivation {
   name = "libyaml-${version}";
 
   src = fetchurl {
     url = "https://pyyaml.org/download/libyaml/yaml-${version}.tar.gz";
-    sha256 = "1karpcfgacgppa82wm2drcfn2kb6q2wqfykf5nrhy20sci2i2a3q";
+    sha256 = "0a87931cx5m14a1x8rbjix3nz7agrcgndf4h392vm62a4rby9240";
   };
 
   meta = with stdenv.lib; {