Fixes test_virtualenv and test_venv failing, closes kivy#1994 (kivy#1995)

Recent pep517 release seems to break tests.

Author: AndreMiras

setup.py

@@ -23,7 +23,7 @@
 install_reqs = [
     'appdirs', 'colorama>=0.3.3', 'jinja2', 'six',
     'enum34; python_version<"3.4"', 'sh>=1.10; sys_platform!="nt"',
-    'pep517', 'pytoml', 'virtualenv'
+    'pep517<0.7.0"', 'pytoml', 'virtualenv'
 ]
 # (pep517, pytoml and virtualenv are used by pythonpackage.py)
 

tests/test_pythonpackage_basic.py

@@ -303,7 +303,7 @@ def test_virtualenv(self):
             ])
             subprocess.check_output([
                 os.path.join(test_dir, "virtualenv", "bin", "pip"),
-                "install", "-U", "pep517"
+                "install", "-U", "pep517<0.7.0"
             ])
             sys_python_path = self.run__get_system_python_executable(
                 os.path.join(test_dir, "virtualenv", "bin", "python")
@@ -336,7 +336,7 @@ def test_venv(self):
             ])
             subprocess.check_output([
                 os.path.join(test_dir, "venv", "bin", "pip"),
-                "install", "-U", "pep517"
+                "install", "-U", "pep517<0.7.0"
             ])
             sys_python_path = self.run__get_system_python_executable(
                 os.path.join(test_dir, "venv", "bin", "python")