Define minimal pyparsing version

[xavfernandez]

closes #91

[xavfernandez]

@dstufft: Ok, you've got the choice between this PR or #93 depending on if you wish a changelog or not ^^

[xavfernandez]

Or even going with something like #98:

--- a/packaging/requirements.py
+++ b/packaging/requirements.py
@@ -56,7 +56,7 @@ _VERSION_SPEC.setParseAction(lambda s, l, t: t._raw_spec or '')
 VERSION_SPEC = originalTextFor(_VERSION_SPEC)("specifier")
 VERSION_SPEC.setParseAction(lambda s, l, t: t[1])

-MARKER_EXPR = originalTextFor(MARKER_EXPR())("marker")
+MARKER_EXPR = originalTextFor(MARKER_EXPR(None))("marker")
 MARKER_EXPR.setParseAction(
     lambda s, l, t: Marker(s[t._original_start:t._original_end])
 )

Which should/could work with pyparsing==2.0.1