habitat: some pip-installed Python modules give a CERTIFICATE_VERIFY_FAILED error during build

For example:

    Download error on https://pypi.python.org/simple/numpy/: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.
c:590) -- Some packages may not be found!
    Couldn't find index page for 'numpy' (maybe misspelled?)
    Download error on https://pypi.python.org/simple/: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)
 -- Some packages may not be found!
    No local packages or download links found for numpy>=1.6
    Traceback (most recent call last):
      File "<string>", line 1, in <module>

      File "/tmp/pip-build-j0adA9/numexpr/setup.py", line 219, in <module>
        setup_package()
      File "/tmp/pip-build-j0adA9/numexpr/setup.py", line 215, in setup_package
        setup(**metadata)
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/distutils/core.py", line 111, in setup
        _setup_distribution = dist = klass(attrs)
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/setuptools/dist.py", line 268, in __init_
_
        self.fetch_build_eggs(attrs['setup_requires'])
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/setuptools/dist.py", line 313, in fetch_b
uild_eggs
        replace_conflicting=True,
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/pkg_resources/__init__.py", line 836, in
resolve
        dist = best[req.key] = env.best_match(req, ws, installer)
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1081, in
 best_match
        return self.obtain(req, installer)
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1093, in
 obtain
        return installer(requirement)
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/setuptools/dist.py", line 380, in fetch_b
uild_egg
        return cmd.easy_install(req)
      File "/hab/pkgs/core/python2/2.7.11/20160620182552/lib/python2.7/site-packages/setuptools/command/easy_install.py", line
 623, in easy_install
        raise DistutilsError(msg)
    distutils.errors.DistutilsError: Could not find suitable distribution for Requirement.parse('numpy>=1.6')

Relevant plan.sh lines (this is a build plan embedded at private_repo/habitat/plan.sh):

pkg_deps=(core/glibc core/gcc core/python2)
pkg_build_deps=(core/gcc core/make core/python2 core/git)
pkg_bin_dirs=(bin)
pkg_lib_dirs=(lib)
pkg_include_dirs=(include)

do_download(){
  return 0
}

do_verify(){
  return 0
}

do_unpack(){
  return 0
}

do_build(){
  pushd /src > /dev/null
  pwd
  pip install --upgrade pip
  pip install --install-option="--prefix=${pkg_prefix}" $(grep numpy requirements.txt)
  pip install --install-option="--prefix=${pkg_prefix}" $(grep numexpr requirements.txt)
  pip install --install-option="--prefix=${pkg_prefix}" $(grep Cython requirements.txt)
  pip install --install-option="--prefix=${pkg_prefix}" -r requirements.txt
  popd > /dev/null
}

do_install(){
  pushd $PLAN_CONTEXT > /dev/null
  pip install -e .
  popd > /dev/null
}