Define license on pypi

[Kreyren]

Define the license for this project on pypi so that it can be automated e.g. package import by guix users

kreyren@dreamon ~$ guix import pypi better-exceptions

Starting download of /tmp/guix-file.HIExkY
From https://files.pythonhosted.org/packages/26/d8/30b745b965765c08ee132fd590fca46c31296e8f1a606de0c53cc6b5a68f/better_exceptions-0.3.3.tar.gz...
 …ns-0.3.3.tar.gz  29KiB              2.6MiB/s 00:00 [##################] 100.0%

Starting download of /tmp/guix-file.O3ArzJ
From https://files.pythonhosted.org/packages/ed/50/abf6850135f1e95d321a525d0a36e05255a039b3fc118b7d88413e8a8207/better_exceptions-0.3.3-py3-none-any.whl...
 …3.3-py3-none-any.whl  12KiB         2.6MiB/s 00:00 [##################] 100.0%
guix import: warning: Failed to extract file: better_exceptions-0.3.3.dist-info/METADATA from wheel.
(package
  (name "python-better-exceptions")
  (version "0.3.3")
  (source (origin
            (method url-fetch)
            (uri (pypi-uri "better_exceptions" version))
            (sha256
             (base32
              "0fs5nfg7xdbx5hb1h1148kax68g9wn0h7cclx3k08psd8hcbrrp4"))))
  (build-system python-build-system)
  (home-page "https://github.com/qix-/better-exceptions")
  (synopsis "Pretty and helpful exceptions, automatically")
  (description "Pretty and helpful exceptions, automatically")
  # required field (license license:expat) missing due to it not being defined in pypi

[Qix-]

PR Welcome :)