python: Make .isPyPy flag more accurate.

nix-repl> map (s: s.isPyPy) [ python python3 pypy pypy3 ]
[ false false true true ]

lib/default.nix

@@ -81,7 +81,7 @@ let
       intersperse concatStringsSep concatMapStringsSep
       concatImapStringsSep makeSearchPath makeSearchPathOutput
       makeLibraryPath makeBinPath optionalString
-      hasPrefix hasSuffix stringToCharacters stringAsChars escape
+      hasInfix hasPrefix hasSuffix stringToCharacters stringAsChars escape
       escapeShellArg escapeShellArgs replaceChars lowerChars
       upperChars toLower toUpper addContextFrom splitString
       removePrefix removeSuffix versionOlder versionAtLeast getVersion

pkgs/development/interpreters/python/default.nix

@@ -30,7 +30,7 @@ with pkgs;
         isPy2 = lib.strings.substring 0 1 pythonVersion == "2";
         isPy3 = lib.strings.substring 0 1 pythonVersion == "3";
         isPy3k = isPy3;
-        isPyPy = interpreter == "pypy";
+        isPyPy = lib.hasInfix "pypy" interpreter;
 
         buildEnv = callPackage ./wrapper.nix { python = self; inherit (pythonPackages) requiredPythonModules; };
         withPackages = import ./with-packages.nix { inherit buildEnv pythonPackages;};