gnu: python-pytest: Update to 3.5.0.

* gnu/packages/python.scm (python-six): Replace system* with invoke.
(python-six-bootstrap, python2-six-bootstrap, python2-funcsigs-bootstrap): New
variables.
(python-more-itertools)[propagated-inputs]: Replace python-six with
python-six-bootstrap.
* gnu/packages/check.scm (python-pytest): Update to 3.5.0.
[propagated-inputs]: Add python-attrs-bootstrap,
python-more-itertools-bootstrap, python-pluggy and python-six-bootstrap.
[properties]: Add python2-variant.
(python2-pytest)[propagated-inputs]: Add python2-funcsigs.
(python-pytest-bootstrap)[properties]: Add python2-variant.
(python2-pytest-bootstrap)[propagated-inputs]: Add python2-funcsigs-bootstrap.

gnu/packages/python.scm

@@ -1157,6 +1157,16 @@ Python file, so it can be easily copied into your project.")
 (define-public python2-six
   (package-with-python2 python-six))
 
+(define-public python-six-bootstrap
+  (package
+    (inherit python-six)
+    (name "python-six-bootstrap")
+    (native-inputs `())
+    (arguments `(#:tests? #f))))
+
+(define-public python2-six-bootstrap
+  (package-with-python2 python-six-bootstrap))
+
 (define-public python-schedule
   (package
     (name "python-schedule")
@@ -1713,6 +1723,15 @@ matching them against a list of media-ranges.")
      "Backport of @code{funcsigs} which was introduced in Python 3.3.")
     (license license:asl2.0)))
 
+(define-public python2-funcsigs-bootstrap
+  (package
+    (inherit python2-funcsigs)
+    (name "python2-funcsigs-bootstrap")
+    (native-inputs `())
+    (arguments
+     `(#:tests? #f
+       ,@(package-arguments python2-funcsigs)))))
+
 (define-public python-pafy
   (package
     (name "python-pafy")
@@ -13175,7 +13194,7 @@ file system events on Linux.")
          "0i3ch700g5fyjp692gprlnzbysl8w0sa2vijbp3s40drvk67xkn9"))))
     (build-system python-build-system)
     (propagated-inputs
-     `(("python-six" ,python-six)))
+     `(("python-six" ,python-six-bootstrap)))
     (home-page "https://github.com/erikrose/more-itertools")
     (synopsis "More routines for operating on iterables, beyond itertools")
     (description "Python's built-in @code{itertools} module implements a