bug#68967: guix import: Issues handling Unicode

[Tomás Ortín]

"guix import" seems to have issues when importing text encoded in some 
non-ascii characters (or at least in Chinese).
As an example to reproduce, this is the output generated by "guix import 
pypinyin":

```
(package
  (name "python-pypinyin")
  (version "0.50.0")
  (source
   (origin
     (method url-fetch)
     (uri (pypi-uri "pypinyin" version))
     (sha256
      (base32 "1yw075j7hjp1kc1jdihp4yq3jd5jpp7grzm5mh1d3fkqkd5prrvj"))))
  (build-system pyproject-build-system)
  (propagated-inputs (list python-enum34 python-typing))
  (home-page "https://github.com/mozillazg/python-pinyin";)
  (synopsis "汉字拼音转换模块/工具.")
  (description "汉字拼音转换模块/工具.")
  (license license:expat))
```