bug#34266: pypi importer cannot handle [ and ] correctly

[swedebugia <swedebugia@riseup.net>]

$ ./pre-inst-env guix import pypi beaker

following redirection to `https://pypi.org/pypi/Beaker/json'...

Starting download of /tmp/guix-file.p15GJZ
 From 
https://files.pythonhosted.org/packages/c2/21/b052b2fbfee3def06670923d5d34b0d353d4c278013e4a714c3fb663f150/Beaker-1.10.0.tar.gz...
  ...0.0.tar.gz  40KiB                 521KiB/s 00:00 
[##################] 100.0%
(package
   (name "python-beaker")
   (version "1.10.0")
   (source
     (origin
       (method url-fetch)
       (uri (pypi-uri "beaker" version))
       (sha256
         (base32
           "0l047yl3n9b3w7ba0wrqdb5fpww5y8pjy20kah2mlpr230lqjwk0"))))
   (build-system python-build-system)
   (propagated-inputs
     `(("python-[crypto]" ,#{python-\x5b;crypto\x5d;}#)
       ("python-[cryptography]"
        ,#{python-\x5b;cryptography\x5d;}#)
       ("python-[pycrypto]"
        ,#{python-\x5b;pycrypto\x5d;}#)
       ("python-[pycryptodome]"
        ,#{python-\x5b;pycryptodome\x5d;}#)
       ("python-[testsuite]"
        ,#{python-\x5b;testsuite\x5d;}#)
       ("python-coverage" ,python-coverage)
       ("python-cryptography" ,python-cryptography)
       ("python-cryptography" ,python-cryptography)
       ("python-funcsigs" ,python-funcsigs)
       ("python-memcached" ,python-memcached)
       ("python-mock" ,python-mock)
       ("python-nose" ,python-nose)
       ("python-pycrypto" ,python-pycrypto)
       ("python-pycryptodome" ,python-pycryptodome)
       ("python-pycryptodome" ,python-pycryptodome)
       ("python-pycryptopp" ,python-pycryptopp)
       ("python-pylibmc" ,python-pylibmc)
       ("python-pymongo" ,python-pymongo)
       ("python-redis" ,python-redis)
       ("python-sqlalchemy" ,python-sqlalchemy)
       ("python-webtest" ,python-webtest)))
   (home-page "https://beaker.readthedocs.io/")
   (synopsis
     "A Session and Caching library with WSGI Middleware")
   (description
     "A Session and Caching library with WSGI Middleware")
   (license license:bsd-3))