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.