exception from parsing requirements

[zooko]

as reported by ambimorph on IRC:

amber@abhyasa:~/tahoe-lafs$ bin/tahoe @python-coverage run --branch --include='src/allmydata/*' @tahoe debug trial allmydata.test.test_cli
Traceback (most recent call last):
  File "/usr/bin/python-coverage", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/home/amber/tahoe-lafs/support/lib/python2.7/site-packages/setuptools-0.6c16dev4.egg/pkg_resources.py", line 2638, in <module>
    working_set.add(dist)
  File "/home/amber/tahoe-lafs/support/lib/python2.7/site-packages/setuptools-0.6c16dev4.egg/pkg_resources.py", line 532, in add
    for thisreq in parse_requirements(thisreqstr):
  File "/home/amber/tahoe-lafs/support/lib/python2.7/site-packages/setuptools-0.6c16dev4.egg/pkg_resources.py", line 2451, in parse_requirements
    raise ValueError("Missing distribution spec", line)
ValueError: ('Missing distribution spec', '=', {'thisreqstr': '='})

Investigation showed the the __requires___ variable held a string ("coverage==3.4" instead of an iterable containing strings, therefor ​this code in our splinter of setuptools tried to parse each character of that string.