osbuild: replace capture_output in subprocess.run

The `capture_output` argument for subprocess.run was added in 3.7,
but want to support 3.6 as well. Change all the usages of it with
`stdout=subprocess.PIPE` that will have the same effect, at least
for stdout.

assemblers/org.osbuild.oci-archive

@@ -103,7 +103,7 @@ MEDIA_TYPES = {
 
 def sha256sum(path: str) -> str:
     ret = subprocess.run(["sha256sum", path],
-                         capture_output=True,
+                         stdout=subprocess.PIPE,
                          encoding="utf-8",
                          check=True)
 

test/mod/test_util_ostree.py

@@ -78,7 +78,7 @@ class TestObjectStore(unittest.TestCase):
                      "tree",
                      "--print-only",
                      path],
-                    capture_output=True)
+                    stdout=subprocess.PIPE)
             self.assertEqual(r.returncode, 0)
             js = json.loads(r.stdout)
 

test/test.py

@@ -176,7 +176,7 @@ class TestBase(unittest.TestCase):
         try:
             r = subprocess.run(["rpm-ostree", "--version"],
                                encoding="utf-8",
-                               capture_output=True,
+                               stdout=subprocess.PIPE,
                                check=False)
         except FileNotFoundError:
             return False