requirements: relax version bound on "pep517" (kivy#2680)

The "<0.7.0" bound was added in kivy@9f6d6fc
to fix kivy#1994 :

> The `TestGetSystemPythonExecutable.test_virtualenv` and `TestGetSystemPythonExecutable.test_venv` tests started failing all of a sudden. Error was:
>
> ```
> ModuleNotFoundError: No module named \'pytoml\'\n'
> ```
>
> This ca be reproduced in local via:
>
> ```shell
> pytest tests/test_pythonpackage_basic.py::TestGetSystemPythonExecutable::test_virtualenv
> ```

I think this no longer applies,
`$ pytest tests/test_pythonpackage_basic.py::TestGetSystemPythonExecutable`
runs successfully for me, using latest `pep517==0.13.0`.

setup.py

@@ -22,7 +22,7 @@
 install_reqs = [
     'appdirs', 'colorama>=0.3.3', 'jinja2',
     'sh>=1.10; sys_platform!="nt"',
-    'pep517<0.7.0', 'toml',
+    'pep517', 'toml',
 ]
 # (pep517 and toml are used by pythonpackage.py)
 

tests/test_pythonpackage_basic.py

@@ -300,7 +300,7 @@ def test_venv(self):
             ])
             subprocess.check_output([
                 os.path.join(test_dir, "venv", "bin", "pip"),
-                "install", "-U", "pep517<0.7.0"
+                "install", "-U", "pep517"
             ])
             subprocess.check_output([
                 os.path.join(test_dir, "venv", "bin", "pip"),