Use Popen rather than run()

scripts/templating/matrix_templates/units.py

@@ -838,21 +838,21 @@ class MatrixUnits(Units):
             tc_lines = []
             if os.path.isdir(tc_path):
                 logger.info("Generating towncrier changelog for: %s" % name)
-                p = subprocess.run(
+                p = subprocess.Popen(
                     ['towncrier', '--version', 'Unreleased Changes', '--name', name, '--draft'],
                     cwd=tc_path,
-                    check=False, # We'll manually check the exit code
                     stderr=subprocess.PIPE,
                     stdout=subprocess.PIPE,
                 )
+                stdout, stderr = p.communicate()
                 if p.returncode != 0:
                     # Something broke - dump as much information as we can
                     logger.error("Towncrier exited with code %s" % p.returncode)
-                    logger.error(p.stdout.decode('UTF-8'))
-                    logger.error(p.stderr.decode('UTF-8'))
+                    logger.error(stdout.decode('UTF-8'))
+                    logger.error(stderr.decode('UTF-8'))
                     raw_log = ""
                 else:
-                    raw_log = p.stdout.decode('UTF-8')
+                    raw_log = stdout.decode('UTF-8')
 
                     # This is a bit of a hack, but it does mean that the log at least gets *something*
                     # to tell us it broke