CI: Flakey Test
https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=22330

Below test fails when ResourceWarnings are raised

```
______________ test_chunks_have_consistent_numerical_type[python] ______________
[gw1] linux -- Python 3.6.1 /home/vsts/miniconda3/envs/pandas-dev/bin/python

all_parsers = <pandas.tests.io.parser.conftest.PythonParser object at 0x7f204e3382b0>

    def test_chunks_have_consistent_numerical_type(all_parsers):
        parser = all_parsers
        integers = [str(i) for i in range(499999)]
        data = "a\n" + "\n".join(integers + ["1.0", "2.0"] + integers)
    
        # Coercions should work without warnings.
        with tm.assert_produces_warning(None):
>           result = parser.read_csv(StringIO(data))

pandas/tests/io/parser/test_common.py:1204: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <contextlib._GeneratorContextManager object at 0x7f2010677550>
type = None, value = None, traceback = None

    def __exit__(self, type, value, traceback):
        if type is None:
            try:
>               next(self.gen)
E               AssertionError: Caused unexpected warning(s): [('ResourceWarning', ResourceWarning("unclosed file <_io.BufferedReader name='testmultiindex.xlsm'>",), '/home/vsts/work/1/s/pandas/io/parsers.py', 2931), ('ResourceWarning', ResourceWarning("unclosed file <_io.BufferedReader name='testmultiindex.xlsx'>",), '/home/vsts/work/1/s/pandas/io/parsers.py', 2931), ('ResourceWarning', ResourceWarning("unclosed file <_io.BufferedReader name='test_index_name_pre17.xlsm'>",), '/home/vsts/work/1/s/pandas/io/parsers.py', 2931), ('ResourceWarning', ResourceWarning("unclosed file <_io.BufferedReader name='test1.xlsx'>",), '/home/vsts/work/1/s/pandas/io/parsers.py', 2931), ('ResourceWarning', ResourceWarning("unclosed file <_io.BufferedReader name='test4.xlsx'>",), '/home/vsts/work/1/s/pandas/io/parsers.py', 2931)]
```