Make pypi2nix work with a recent nixpkgs

The requirements.nix generated by pypi2nix doesn't work with recent nixpkgs:

error: attribute 'patchPhase' missing, at /Users/dhess/git/streamalert/requirements.nix:26:24

This error is relatively easily fixed in the generated requirements.nix, but then it fails again when trying to build python2.7-bootstrapped-pip-19.2.3:

...
configuring
no configure script, doing nothing
building
no Makefile, doing nothing
installing
/nix/store/fc78qmli2r4zkdfzc7sr1m6s8m767d6x-stdenv-darwin/setup: line 1339: /nix/store/lashbdzkfrwdcj7n1y5rqz24q4vknx5x-python2.7-bootstrapped-pip-19.2.3/bin/pip: No such file or directory
builder for '/nix/store/skv4aghvfhyj3rn3l4naj68ki75ilnsz-python2.7-bootstrapped-pip-19.2.3.drv' failed with exit code 1
error: build of '/nix/store/skv4aghvfhyj3rn3l4naj68ki75ilnsz-python2.7-bootstrapped-pip-19.2.3.drv' on 'ssh://remote-builder@mac-builder-a.prod.dev.hackworth-corp.com' failed: builder for '/nix/store/skv4aghvfhyj3rn3l4naj68ki75ilnsz-python2.7-bootstrapped-pip-19.2.3.drv' failed with exit code 1
builder for '/nix/store/skv4aghvfhyj3rn3l4naj68ki75ilnsz-python2.7-bootstrapped-pip-19.2.3.drv' failed with exit code 1
cannot build derivation '/nix/store/6gmqjjvv7yzknq0bxz4ragsqshwr6fmx-python2.7-pip-19.2.3.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/ckdj141vqw6h7vn55pjrkqsl7x2r6miv-python2.7-setuptools-41.2.0.drv': 1 dependencies couldn't be built
error: build of '/nix/store/091ky9ckzg8984gn94mg73bx146pvypn-python2.7-aliyun-python-sdk-core-2.13.5.drv', '/nix/store/4j7ydfmd0srpy8s2zldw5bw6dym0y9b3-python2.7-netaddr-0.7.19.drv', '/nix/store/6npq1ic3371qqa970knha06f6nrq8dr1-python2.7-nose-1.3.7.drv', '/nix/store/7md31ah3r016dqlf86jnbygfpal6bk4v-python2.7-cbapi-1.5.1.drv', '/nix/store/8w0dgqljpad8nacjb659r7f0h9b2smli-python2.7-google-api-python-client-1.6.4.drv', '/nix/store/9z58aa7g5hm2din7nf800ym2n0c86d6c-python-catch-conflicts-hook.drv', '/nix/store/ammm36h8cpin22945fnc32xqv1fm5wrc-setuptools-setup-hook.drv', '/nix/store/b57w4jaj4h0vwkypf8kx0f4kk67ly1xm-python2.7-boxsdk-2.0.0a11.drv', '/nix/store/c7hyx5h5h8dgp2a2z8v9rm0gmfs6fhqg-python2.7-pyfakefs-3.6.drv', '/nix/store/cfr3j0c5iyrhcaf5amyapljf1djz3h3s-python2.7-oauth2client-4.1.3.drv', '/nix/store/ckdj141vqw6h7vn55pjrkqsl7x2r6miv-python2.7-setuptools-41.2.0.drv', '/nix/store/dzrfrlv7r304hlzp2ahmaalhka7ndp6i-python2.7-nose-timer-0.7.5.drv', '/nix/store/hsz0hg4nv1firwq4zh8c9z11566qqg6b-python2.7-backoff-1.6.0.drv', '/nix/store/j1fxasi9bhx45x7s3ha6bnd90anbql4z-python2.7-boto3-1.9.208.drv', '/nix/store/jm9rf14gng55ps8f8l3fdn3amhajh0y1-pip-install-hook.drv', '/nix/store/kx0vp97c3ldh13yv0i1y8zk7hp8x79gs-python2.7-jsonlines-1.2.0.drv', '/nix/store/mxqakxvv39zyndpn5whm1913kv97kyc5-python2.7-moto-1.3.13.drv', '/nix/store/p6visb3g40hf2m6m9blgz310b6c916gi-python2.7-mock-3.0.5.drv', '/nix/store/v4fiwksj8x5938hp2yh1szhylqd0zyii-setuptools-check-hook.drv', '/nix/store/zyhyjx3i9v98ahhv5h6vs7ngbik826nz-python2.7-aliyun-python-sdk-actiontrail-2.0.0.drv' failed