gnu: python-babel: Disable tests that are incompatible with Pytest 4.

* gnu/packages/python-xyz.scm (python-babel)[arguments]: Expand pytest invokation.

gnu/packages/python-xyz.scm

@@ -302,7 +302,14 @@ by @code{binstar}, @code{binstar-build} and @code{chalmers}.")
     (arguments
      `(#:phases (modify-phases %standard-phases
                   (replace 'check
-                    (lambda _ (invoke "pytest" "-vv"))))))
+                    (lambda _
+                      (invoke "pytest" "-vv" "-k"
+                              (string-append
+                               ;; XXX: These tests fail when using Pytest 4.x and
+                               ;; Babel 2.6.0.  Try removing this for later versions.
+                               "not test_no_inherit_metazone_marker_never_in_output"
+                               " and not test_smoke_dates"
+                               " and not test_smoke_numbers")))))))
     (home-page "http://babel.pocoo.org/")
     (synopsis
      "Tools for internationalizing Python applications")