Creating a GUI with Dropdown Menus

Now you have a converter handy for converting MPa to psi. What if you want to convert between other units of pressure? Instead of hard-coding MPa and psi, you want to be able to give the user an OptionMenu of all of the available pressures from the Pint library. You will create a list of all available pressure units, then make two OptionMenus to let the user pick which pressures to convert between. The GUI will now look like this:

	Column 1	Column 2	Column 3
Row 1		Label saying “Converter”
Row 2	Label saying “Convert”	Entry field for inputting a str	OptionMenu with all pressures
Row 3		Label saying “into”

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