If we don't set this we are mixing python versions in the same package.
Any package called from `python-packages.nix` using `pythonPackages` directly as an argument is currently wrong.
Before:
```
nix-repl> python3Packages.bugseverywhere.buildInputs
[ «derivation /nix/store/csd0d2j3vgl5dnag6afi5q92xb64rv7z-python2.7-Jinja2-2.10.1.drv» «derivation /nix/store/gsj389hprxlr8a39b0ccszagirirf1lv-python2.7-cherrypy-17.4.1.drv» «derivation /nix/store/ysmd4wdkbi9gd6bw2ha92hwrd2k2xlnh-python2.7-bootstrapped-pip-19.0.3.drv» ]
```
After:
```
nix-repl> python3Packages.bugseverywhere.buildInputs
[ «derivation /nix/store/m0mfx1i98ciby8mx8xjva98kinhqkj6r-python3.7-Jinja2-2.10.1.drv» «derivation /nix/store/2gqd0sbiwqmin2814ja83pnyal8pq86a-python3.7-cherrypy-18.1.1.drv» «derivation /nix/store/0rq8n7xq2ksxiq2y2d844i9516p0rd41-python3.7-bootstrapped-pip-19.0.3.drv» ]
```
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