fix missing revnum attribute on pure-python Satrec

[plentydone]

Failure looks like this:

ERROR: test_export_tle_sgp4init (sgp4.tests.TestFunctions.test_export_tle_sgp4init)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/andrew/python-sgp4/.tox/py312/lib/python3.12/site-packages/sgp4/tests.py", line 252, in test_export_tle_sgp4init
    outline1, outline2 = export_tle(sat)
                         ^^^^^^^^^^^^^^^
  File "/home/andrew/python-sgp4/.tox/py312/lib/python3.12/site-packages/sgp4/exporter.py", line 89, in export_tle
    append(str(satrec.revnum).rjust(5))
               ^^^^^^^^^^^^^
AttributeError: 'Satrec' object has no attribute 'revnum'. Did you mean: 'elnum'?

Let me know if any adjustments are needed.