Refactor mkPython.

Signed-off-by: Rolf Schröder <rolf.schr@gmail.com>

Author: rolfschr

etc/nix/flake.nix

@@ -61,18 +61,28 @@
           pypiDataSha256 =
             "0499zl39aia74f0i7fkn5dsy8244dkmcw4vzd5nf4kai605j2jli";
         });
+      # This wrapper allows to setup both the production as well as the
+      # development Python environments in the same way (albeit having
+      # different requirements.txt).
+      getPythonEnv = system: requirements:
+        machnixFor.${system}.mkPython {
+          requirements = ''
+            ${requirements}
+          '';
+          # Fix an issue with an upstream dep of GitPython.
+          # https://github.com/DavHau/mach-nix/issues/287
+          # See https://github.com/DavHau/mach-nix/issues/318
+          _.gitpython.propagatedBuildInputs.mod = pySelf: self: oldVal:
+            oldVal ++ [ pySelf.typing-extensions ];
+        };
 
     in {
 
       # A Nixpkgs overlay.
       overlay = final: prev:
         with final.pkgs; {
 
-          pythonEnv = machnixFor.${system}.mkPython {
-            requirements = ''
-              ${requirements}
-            '';
-          };
+          pythonEnv = getPythonEnv system requirements;
 
           vulnerablecode = stdenv.mkDerivation {
             inherit version;
@@ -117,12 +127,10 @@
       # Tests run by 'nix flake check' and by Hydra.
       checks = forAllSystems (system:
         let
-          pythonEnvDev = machnixFor.${system}.mkPython {
-            requirements = ''
-              ${requirements}
-              ${requirementsDev}
-            '';
-          };
+          pythonEnvDev = getPythonEnv system ''
+            ${requirements}
+            ${requirementsDev}
+          '';
 
         in {
           inherit (self.packages.${system}) vulnerablecode;