Doc: improve python bdist_wheel fix

doc/languages-frameworks/python.md

@@ -767,21 +767,25 @@ in newpkgs.python27.withPackages (ps: [ps.django_guardian ])
 
 ### `python setup.py bdist_wheel` cannot create .whl
 
-Executing `python setup.py bdist_wheel` fails with
+Executing `python setup.py bdist_wheel` in a `nix-shell `fails with
 ```
 ValueError: ZIP does not support timestamps before 1980
 ```
 This is because files are included that depend on items in the Nix store which have a timestamp of, that is, it corresponds to January the 1st, 1970 at 00:00:00. And as the error informs you, ZIP does not support that.
-Fortunately `bdist_wheel` takes into account `SOURCE_DATE_EPOCH`. On Nix this value is set to 1. By setting it to a value correspond to 1980 or later it is possible to build wheels.
+The command `bdist_wheel` takes into account `SOURCE_DATE_EPOCH`, and `nix-shell` sets this to 1. By setting it to a value corresponding to 1980 or later, or by unsetting it, it is possible to build wheels.
 
 Use 1980 as timestamp:
 ```
-SOURCE_DATE_EPOCH=315532800 python3 setup.py bdist_wheel
+nix-shell --run "SOURCE_DATE_EPOCH=315532800 python3 setup.py bdist_wheel"
 ```
 or the current time:
 ```
-SOURCE_DATE_EPOCH=$(date +%s) python3 setup.py bdist_wheel
+nix-shell --run "SOURCE_DATE_EPOCH=$(date +%s) python3 setup.py bdist_wheel"
 ```
+or unset:
+"""
+nix-shell --run "unset SOURCE_DATE_EPOCH; python3 setup.py bdist_wheel"
+"""
 
 ### `install_data` / `data_files` problems