Fix typing annotation of MySQLConnectionAbstract's close function

[parulgupta1004]

The annotation for 'close' function in 'MySQLConnectionAbstract' is incorrect which throws error when run through a type-checker. Thsi PR corrects the typing of the function.

[mysql-oca-bot]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/
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Thanks

[parulgupta1004]

I have signed OCA. Can I get a review for this PR please?

[oscpache]

Hey Parul,

The patch looks good, it will be applied, and it should go out in the next release.

Thanks for your contribution!

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
"I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it."
Thanks

[parulgupta1004]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=117347 for updates.
Thanks