Replace type checking method

[leviable]

Suppose I was to subclass the Test class and add attributes specific to my environment:

from testrail.test import Test as TestBase

class Test(TestBase):
    def __init__(self, testrail_test):
        if not isinstance(testrail_test, TestBase):
            raise ValueError("testrail_test must be of TestRail Test type")
        super().__init__(content=testrail_test.raw_data())

    @property
    def long_running(self):
        return self._content.get("long_running", False)

    @long_running.setter
    def long_running(self, val):
        self._content['long_running'] = val

If I then instantiate one of these new tests and attempt to assign it to a result:

tests = tr_client.tests(run)
test = Test(tests[0])

result = tr_client.result()
result.test = test

An exception will be thrown because of the Results type checking:

    @test.setter
    def test(self, test_obj):
        if type(test_obj) != Test:
            raise TestRailError('input must be a Test')

The type(foo) == Foo idiom isn't capable of handling subclassing, wheres isinstance can:

    @test.setter
    def test(self, test_obj):
        if isintance(test_obj, Test):
            raise TestRailError('input must be a Test')

This PR reeplaces the method used for type checking (type(foo) == Foo) with one that allows for subclassing (isinstance(foo, Foo))

[coveralls]

Coverage remained the same at 82.575% when pulling 202d982 on levi-rs:add-better-type-detection into 55250c9 on travispavek:master.