Override from_formula so that extra array args can be passed by name
Test maintenance
BLD: let setuptools/pip handle dependencies that are not installed at all.

Closes gh-1897.  See scipy commit 8e7ee0c4b for a similar change that has
worked well.
BLD: use setup/install_requires only if setuptools is installed.
BLD: fix issue with version strings reported by @cel4 in gh-1897.

[ci skip]
Merge pull request #2000 from jseabold/build-deps

BLD: Let pip/setuptools handle dependencies that aren't installed at all.
BLD: Do not generate cython on clean. Closes #1852.
Merge pull request #2006 from jseabold/fix-cythonize

BLD: Do not generate cython on clean. Closes #1852.
BLD: Include README in sdist
BLD: Check for dependencies unconditionally. Closes #1897.
Merge pull request #2007 from jseabold/fix-build-issues

BLD: Fix build issues
Merge pull request #1954 from kshedden/phreg_formula

ENH: PHReg formula improvements
FIXUP/WIP: Refactor to use generic data handling
ENH: Allow more general param_names in data handling
ENH: Allow attachment of generic names.
ENH: Refactor to allow wrappers to work.
ENH: Handle the generic 2d case.
ENH: Wrap 2d attributes
REF: Return DataFrame instead of dict.
TST: Fix tests for refactor