While working on <https://hydra.gnu.org/26818>, I tried updating
python-mock to 2.0.0, the latest release.

However, a large number of dependent packages failed to build [0].

Many of the failures happen because python-six is not found; the
ChangeLog distributed with mock 2.0.0 points out that six is used in a
different way with mock 2.0.0.

But, there is a dependency cycle between mock and six! So, we could try
to create "minimal" package variants to bootstrap these packages, or we
could try adding six to the build environments of the failing packages.

What do you think?

[0] These failed to build with python-mock@2:
/gnu/store/03cr4n55nl7yk7q9y77w3vghg3j4z9rr-pybitmessage-0.6.1.drv
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/gnu/store/1djj6drsdfp95dlfyairzhynq0rk76cz-postorius-1.0.3.drv
/gnu/store/1k2cr1kzbdcx5mdwi5wk6gjdnzlq59hh-python2-mpd2-0.5.5.drv
/gnu/store/20hncyk2sj05f2h8q7m5c0li9v3x95sk-python2-sphinx-cloud-sptheme-1.8.0.drv
/gnu/store/23b1zmm9j7akr78nwhk3i1i9gzdszf8y-python2-pykka-1.2.1.drv
/gnu/store/2l4jnbvggnxj2v003jfi93v19dr3d2nj-python2-faker-0.7.9.drv
/gnu/store/2pr940l4zvqfj3p8hqr6cz48y952vlcj-pepr-1.0.9.drv
/gnu/store/35dj7hy1dhgsy793rb69bl262kfwingk-fio-2.19.drv
/gnu/store/3l8bh3c6c13n9qwbqwpxfd0bg8yr0r7k-python2-bcrypt-3.1.0.drv
/gnu/store/44 2j75kh1vlf9mxf24nwm1sdk6jch2sk-sshuttle-0.78.1.drv
/gnu/store/4gi9l1rc2ldl363h3qd5i6fqnin5x5yi-awscli-1.11.63.drv
/gnu/store/4mqhl08hh0rviyykgfqz7y55vjdfmvl5-python-pykka-1.2.1.drv
/gnu/store/4yaqbdvrwnj5qqmyf63j23pnvaylizkm-alot-0.4.drv
/gnu/store/50y8ywndhx5a83sm1dcw5cvd4sc4pj62-python2-ipywidgets-5.2.2.drv
/gnu/store/5i46sfxz9anjp6114c81cwypxcfzl81x-python2-ndg-httpsclient-0.4.2.drv
/gnu/store/5npy61z5bz2vx9yr7f5vc5v1ih2ykbas-python-cachecontrol-0.11.6.drv
/gnu/store/5xbxarzphxbyjls9dsipncwd53x2cb6d-python2-reno-2.0.3.drv
/gnu/store/674h107gf6vd0f6x8k5is6054i3a6k4g-python-swiftclient-2.6.0.drv
/gnu/store/6x9azxnxiw2h3phpyd18l03dziypykag-duplicity-0.7.12.drv
/gnu/store/70jyzfw0h7gp3abzsfvy0prgpfgj0ykj-deeptools-2.1.1.drv
/gnu/store/7g0azfip3xblbk78lykbv5hdd1m3yjc4-python2-acme-0.14.0.drv
/gnu/store/7jbyyh3pqkk9fwypvmygjbis3iblky8j-python2-git-review-1.25.0.drv
/gnu/store/7qgqacx88la4vz19fhfrys94181i06yv-python2-freezegun-0.3.8.drv
/gnu/store/7s8m70cwbrgkclw9h1gwx808ibfjvqhj-python2-natsort-5.0.2.drv
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/gnu/store/94bhp4lq9rlh0f7d0qiy7jm470wvlvyc-python2-django-simple-math-captcha-1.0.7.drv
/gnu/store/9h163crlikj52bifby0605cb0a09bblj-python2-jupyter-console-5.0.0.drv
/gnu/store/9pppxpfv6465w5sss3lwa0aic4zywz61-python-git-review-1.25.0.drv
/gnu/store/9rmlq9f8l9hgm8qw74lq85p5465zz58h-python2-warpedlmm-0.21.drv
/gnu/store/acnbrx4wgzdbxr8ja5spshmlm58gs3c0-clipper-1.1.drv
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/gnu/store/dsarnw4ac3dj5s46p5ksc3n8wwk24bg2-python2-graphene-0.10.2.drv
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