feat:use context manager to open and close clipboard automatically

[atticus-lv]

maybe a easier way to use it

import clipette

# old
if clipette.open_clipboard():
    clipette.empty_cliboard()
    clipette.set_UNICODETEXT("<some text>")
    clipette.close_clipboard()

# new
with clipette.clipboard():
    clipette.empty_cliboard()
    clipette.set_UNICODETEXT("<some text>")

[b-init]

Using a context manager makes a lot of sense for this. Thanks for the PR.

But testing this made me realize a flaw that existed earlier too, if a non-custom exception is raised after successfully opening the clipboard, it'll halt the program without calling close_clipboard(). It seems that can also be fixed using contextmanagers? Perhaps the current system of having custom exceptions and calling close_clipboard() in them separately can be improved.