Fix pywin.tools.regpy's ModuleNotFoundError: No module named 'dialog'

[Avasam]

Extracted from #2102

Before:

>>> import pywin.tools.regpy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\Python39\lib\site-packages\Pythonwin\pywin\tools\regpy.py", line 3, in <module>
    import dialog
ModuleNotFoundError: No module named 'dialog'

PS C:\Program Files\Python39\Lib\site-packages\pythonwin\pywin\tools> python .\regpy.py
Traceback (most recent call last):
  File "C:\Program Files\Python39\Lib\site-packages\pythonwin\pywin\tools\regpy.py", line 3, in <module>
    import dialog
ModuleNotFoundError: No module named 'dialog'

After:

>>> import pywin.tools.regpy
# (no import error)

[mhammond]

Thanks!