gnu: python-pytest-xdist: Fix by downgrading to 1.34.0, run test suite.

Commit ed72e55831 updated the package to 2.1.0, but this version doesn't work
with Pytest 5, the version currently used in Guix.  Downgrade to the last
version that is compatible with Pytest 5.

* gnu/packages/check.scm (python-pytest-xdist): Downgrade to 1.34.0, with a
comment explaining why.  Strip trailing #t.
[source]: Remove origin modules and snippet field, no longer necessary.
[phases]{check}: Override to run the test suite.
[propagated-inputs]: Remove python-py and add python-pytest-forked.

gnu/packages/check.scm

@@ -1310,38 +1310,32 @@ (define-public python2-pytest-mock
 (define-public python-pytest-xdist
   (package
     (name "python-pytest-xdist")
-    (version "2.1.0")
+    ;; There is an issue that causes all releases after 1.34 to fail on Pytest
+    ;; 5 (see: https://github.com/pytest-dev/pytest-xdist/issues/697).
+    (version "1.34.0")
     (source
      (origin
        (method url-fetch)
        (uri (pypi-uri "pytest-xdist" version))
        (sha256
         (base32
-         "0wh6pn66nncfs6ay0n863bgyriwsgppn8flx5l7551j1lbqkinc2"))
-       (modules '((guix build utils)))
-       (snippet
-        '(begin
-           ;; Remove pre-compiled .pyc files from source.
-           (for-each delete-file-recursively
-                     (find-files "." "__pycache__" #:directories? #t))
-           (for-each delete-file (find-files "." "\\.pyc$"))
-           #t))))
+         "1vh4ps32lp5ignch5adbl3pgchvigdfmrl6qpmhxih54wa1qw3il"))))
     (build-system python-build-system)
     (arguments
-     '(#:tests? #f)) ;FIXME: Some tests are failing.
-       ;; #:phases
-       ;; (modify-phases %standard-phases
-       ;;   (delete 'check)
-       ;;   (add-after 'install 'check
-       ;;     (lambda* (#:key inputs outputs #:allow-other-keys)
-       ;;       (add-installed-pythonpath inputs outputs)
-       ;;       (zero? (system* "py.test" "-v")))))
+     `(#:phases
+       (modify-phases %standard-phases
+         (replace 'check
+           (lambda* (#:key inputs outputs tests? #:allow-other-keys)
+             (when tests?
+               (add-installed-pythonpath inputs outputs)
+               (invoke "pytest" "-vv"
+                       "-n" (number->string (parallel-job-count)))))))))
     (native-inputs
      `(("python-setuptools-scm" ,python-setuptools-scm)))
     (propagated-inputs
      `(("python-execnet" ,python-execnet)
        ("python-pytest" ,python-pytest)
-       ("python-py" ,python-py)))
+       ("python-pytest-forked" ,python-pytest-forked)))
     (home-page
      "https://github.com/pytest-dev/pytest-xdist")
     (synopsis