Hi Guix!

This fixes the python2-tqdm package which failed to build for me since
core-updates was merged:

```
starting phase `check'
running "python setup.py" with command "test" and parameters ()
running test
Searching for functools32
Reading https://pypi.org/simple/functools32/
Download error on https://pypi.org/simple/functools32/: [Errno -2] Name or service not known -- Some packages may not be found!
Couldn't find index page for 'functools32' (maybe misspelled?)
Scanning index of all packages (this may take a while)
Reading https://pypi.org/simple/
Download error on https://pypi.org/simple/: [Errno -2] Name or service not known -- Some packages may not be found!
No local packages or working download links found for functools32
error: Could not find suitable distribution for Requirement.parse('functools32')
command "python" "-c" "import setuptools, tokenize;__file__='setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\\r\\n', '\\n');f.close();exec(compile(code, __file__, 'exec'))" "test" failed with status 1
builder for `/gnu/store/p3v9nnxsk9bl0lxcd2zfzkgx56sv1q11-python2-tqdm-4.19.6.drv' failed with exit code 1
build of /gnu/store/p3v9nnxsk9bl0lxcd2zfzkgx56sv1q11-python2-tqdm-4.19.6.drv failed
View build log at '/var/log/guix/drvs/p3/v9nnxsk9bl0lxcd2zfzkgx56sv1q11-python2-tqdm-4.19.6.drv.bz2'.
guix build: error: build of `/gnu/store/p3v9nnxsk9bl0lxcd2zfzkgx56sv1q11-python2-tqdm-4.19.6.drv' failed
```

Adding python2-functools32 as a native-input fixes the issue.