applying swmm toolkit fix for compatibility with SWIG>=4.3

[aerispaha]

This applies a modification to owa-epanet/wrapper/toolkit.i that resolves an issue that was introduced with SWIG 4.3. This is nearly identical to the fix that was applied to the SWMM toolkit here.

Without this change, swig-wrapped functions that "return None no longer implicitly return void" due to these changes. This seemingly causes functions to return a list with two elements, where the first element is None and the second element is (likely) the object intended to be returned by the function. In practice this results in TypeErrors, for example:

Traceback (most recent call last):
  File "/Users/adamerispaha/code/mws-test/tests/test_run.py", line 38, in test_run_small_inp_downloaded
    self.assertTrue(run_inp(os.path.join(MODELS_DIR, 'small.inp'), collection_interval=120*60))
  File "/Users/adamerispaha/code/mws-test/src/run.py", line 10, in run_inp
    with Network(file_path=file_path, collection_interval=collection_interval) as net:
  File "/Users/adamerispaha/code/mws-test/.venv/lib/python3.10/site-packages/epanetwrapper/network/network.py", line 76, in __enter__
    toolkit.open(self.project, self.inp_file, self.report_file, self.output_file)
  File "/Users/adamerispaha/code/mws-test/.venv/lib/python3.10/site-packages/epanet/toolkit.py", line 671, in open
    return _toolkit.open(ph, inpFile, rptFile, outFile)
TypeError: in method 'open', argument 1 of type 'EN_Project'

The changes to the owa-epanet/wrapper/toolkit.i fixes this behavior.