Fix check_contracts typings

[ChristopherLi05]

Motivation and Context

Addresses #961

Your Changes

Description:

• I added Class = TypeVar("Class", bound=type) as a wildcard to match classes
• I created 2 overload methods for check_contracts that better reflects how the function works

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)

Testing

I tested the following code and PyCharm doesn't show Cannot find reference '__init__' in '(...)->Any'

from python_ta.contracts import check_contracts


@check_contracts
class Foo:
    def __init__(self):
        self.a = 5


@check_contracts
class Bar(Foo):
    def __init__(self):
        Foo.__init__(self)

Checklist

• I have performed a self-review of my own code.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have updated the CHANGELOG.md file.

[david-yz-liu]

@ChristopherLi05 thanks for creating this pull request. Great work! Two comments:

1. You should do a merge/rebase from master; when you do, you'll see in the CHANGELOG that there's a "Bug fixes" section under [unreleased], which is where you should put your new entry. (Version 2.6.2 has already been released.)
2. Please add your name to the list of contributors in the README (assuming you want to be recognized!).

[ChristopherLi05]

Fixed! 😊

[coveralls]

Pull Request Test Coverage Report for Build 6444132922

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 94.593%

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Totals
Change from base Build 6444053842:	0.03%
Covered Lines:	3394
Relevant Lines:	3588

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💛 - Coveralls