run-tests: Use better naming for openpty fds

This is the naming that was proposed in the (unfortunately not adopted)
python/cpython#9100

Author: jepler

tests/run-tests

@@ -87,28 +87,28 @@ def run_micropython(pyb, args, test_file, is_special=False):
                     def get(required=False):
                         rv = b''
                         while True:
-                            ready = select.select([primary], [], [], 0.02)
-                            if ready[0] == [primary]:
-                                rv += os.read(primary, 1024)
+                            ready = select.select([parent_fd], [], [], 0.02)
+                            if ready[0] == [parent_fd]:
+                                rv += os.read(parent_fd, 1024)
                             else:
                                 if not required or rv:
                                     return rv
 
                     def send_get(what):
-                        os.write(primary, what)
+                        os.write(parent_fd, what)
                         return get()
 
                     with open(test_file, 'rb') as f:
                         # instead of: output_mupy = subprocess.check_output(args, stdin=f)
-                        primary, secondary = pty.openpty()
-                        p = subprocess.Popen(args, stdin=secondary, stdout=secondary,
+                        parent_fd, child_fd = pty.openpty()
+                        p = subprocess.Popen(args, stdin=child_fd, stdout=child_fd,
                                              stderr=subprocess.STDOUT, bufsize=0)
                         banner = get(True)
                         output_mupy = banner + b''.join(send_get(line) for line in f)
                         send_get(b'\x04') # exit the REPL, so coverage info is saved
                         p.kill()
-                        os.close(primary)
-                        os.close(secondary)
+                        os.close(parent_fd)
+                        os.close(child_fd)
                 else:
                     output_mupy = subprocess.check_output(args + [test_file], stderr=subprocess.STDOUT)
             except subprocess.CalledProcessError: