python3.pkgs.mistune: write derivation for mistune=2.0.0a4

This version is required for md2gemini program and is incompatible with
mistune=0.8.4

pkgs/development/python-modules/mistune/common.nix

@@ -0,0 +1,18 @@
+{ lib, buildPythonPackage, fetchPypi, nose, version, sha256 }:
+
+buildPythonPackage rec {
+  inherit version;
+  pname = "mistune";
+
+  src = fetchPypi {
+    inherit pname version sha256;
+  };
+
+  buildInputs = [ nose ];
+
+  meta = with lib; {
+    description = "The fastest markdown parser in pure Python";
+    homepage = "https://github.com/lepture/mistune";
+    license = licenses.bsd3;
+  };
+}

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

@@ -1,23 +1,11 @@
-{ lib
-, buildPythonPackage
-, fetchPypi
-, nose
-}:
-
-buildPythonPackage rec {
-  pname = "mistune";
+self: rec {
+  mistune_0_8 = self.callPackage ./common.nix {
     version = "0.8.4";
-
-  src = fetchPypi {
-    inherit pname version;
     sha256 = "59a3429db53c50b5c6bcc8a07f8848cb00d7dc8bdb431a4ab41920d201d4756e";
   };
-
-  buildInputs = [ nose ];
-
-  meta = with lib; {
-    description = "The fastest markdown parser in pure Python";
-    homepage = "https://github.com/lepture/mistune";
-    license = licenses.bsd3;
+  mistune_2_0 = self.callPackage ./common.nix {
+    version = "2.0.0a4";
+    sha256 = "0i6cblmjl58kdmaa21xm0l1ls0kvjpfy45sf73fw3ws6305f628k";
   };
+  mistune = mistune_0_8;
 }

pkgs/top-level/python-packages.nix

@@ -4020,7 +4020,11 @@ in {
 
   mistletoe = callPackage ../development/python-modules/mistletoe { };
 
-  mistune = callPackage ../development/python-modules/mistune { };
+  inherit (import ../development/python-modules/mistune self)
+    mistune
+    mistune_0_8
+    mistune_2_0
+  ;
 
   mitmproxy = callPackage ../development/python-modules/mitmproxy { };