Inject mocks

[rohitsanj]

Implemented patch contextmanager.

• Sample usage of patch:

def test_patch_basic(tb):
    func = tb.ref("func")
    with tb.patch("os.listdir", return_value=['file1', 'file2']) as mock_listdir:
        assert func() == ['file1', 'file2']
        mock_listdir.assert_called_once()

• Sample usage of patch_dict - equivalent to patch.dict of mock library

def test_patch_dict(self, tb):
    with tb.patch_dict("os.environ", {"PATH": "/usr/bin"}, clear=True):
        assert tb.ref("os.environ") == {"PATH": "/usr/bin"} 

Resolves #8

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Codecov Report

Merging #45 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #45   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            7         7           
  Lines          269       284   +15     
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+ Hits           269       284   +15