gnu: python-keyring: Make tests skippable.

* gnu/packages/python-crypto.scm (python-keyring)[arguments]: In custom
'check phase only run tests when tests? is #true.

gnu/packages/python-crypto.scm

@@ -382,8 +382,10 @@ (define-public python-keyring
      `(#:phases
        (modify-phases %standard-phases
          (replace 'check
-           (lambda _
-             (invoke "pytest"))))))
+           (lambda* (#:key tests? #:allow-other-keys)
+             (when tests?
+               (invoke "pytest"))
+             #t)))))
     (native-inputs
      `(("python-toml" ,python-toml)
        ("python-pytest" ,python-pytest)