Stop using pipes, fix write arg type.

[porridge]

Fixes:

.../ledgerhelpers/tests/dogtail/addtrans.py:5: DeprecationWarning: 'pipes' is deprecated and slated for removal in Python 3.13

and

Traceback (most recent call last):
  File ".../ledgerhelpers/tests/dogtail/addtrans.py", line 32, in <module>
    t.write("""
  File "/usr/lib/python3.11/tempfile.py", line 638, in func_wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
TypeError: a bytes-like object is required, not 'str'

This still leaves the following which I have no idea how to fix:

(addtrans:4027391): Gtk-WARNING **: 22:02:06.703: Locale not supported by C library.
	Using the fallback 'C' locale.
dbus[4027379]: arguments to dbus_connection_send_with_reply_and_block() were incorrect, assertion "(error) == NULL || !dbus_error_is_set ((error))" failed in file ../../../dbus/dbus-connection.c line 3557.
This is normally a bug in some application using the D-Bus library.

  D-Bus not built with -rdynamic so unable to print a backtrace
Aborted