TST, fix for issue pandas-dev#17978.

Addition of "hypothesis usage" in test cases of tests/reshape/test_util.py as kind of POC.

Incorporate review comments.
Resolve flake8 warning.
Add section for hypothesis in contributing.rst

Author: sushobhit27

doc/source/contributing.rst

@@ -775,6 +775,66 @@ Tests that we have ``parametrized`` are now accessible via the test name, for ex
    test_cool_feature.py::test_dtypes[int8] PASSED
    test_cool_feature.py::test_series[int8] PASSED
 
+Transitioning to ``hypothesis``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+With the transition to pytest, things have become easier for testing by having reduced boilerplate for test cases and also by utilizing pytest's features like parametizing, skipping and marking test cases.
+However, one has to still come up with input data examples which can be tested against the functionality. There is always a possibility to skip testing an example which could have failed the test case.
+Imagine if some framework could generate random input examples based on the property/specification of the function being tested. That is exactly what hypothesis does by generating the input data based on some set of specifications provided by the user.
+e.g suppose we have to test python's sum function for a list of int.
+
+Here is a sample test case using pytest:
+.. code-block:: python
+    import pytest
+
+    @pytest.mark.parametrize('seq', [
+        [0, 0, 0],
+        [1, 2, 3, 4],
+        [-3, 5, -8, 23],
+        [12345678, 9876543, 567894321]
+    ])
+    def test_sum_using_pytest(seq):
+        total = 0
+        for item in seq:
+            total += item
+        assert sum(seq) == total
+
+output of test cases:
+.. code-block:: shell
+    collecting ... collected 4 items
+    pytest_example.py::test_sum_using_pytest[seq0] PASSED                    [ 25%]
+    pytest_example.py::test_sum_using_pytest[seq1] PASSED                    [ 50%]
+    pytest_example.py::test_sum_using_pytest[seq2] PASSED                    [ 75%]
+    pytest_example.py::test_sum_using_pytest[seq3] PASSED                    [100%]
+
+    ========================== 4 passed in 0.06 seconds ===========================
+
+
+Compare it with below example for the same test case using hypothesis.
+.. code-block:: python
+    from hypothesis import strategies as st
+    from hypothesis import given
+
+
+    @given(st.lists(st.integers()))
+    def test_sum(seq):
+        total = 0
+        for item in seq:
+            total += item
+        assert sum(seq) == total
+
+output of test cases:
+.. code-block:: shell
+    collecting ... collected 1 item
+    hypothesis_example.py::test_sum PASSED                                   [100%]
+
+    ========================== 1 passed in 0.33 seconds ===========================
+
+The main difference in above example is use of a decorator "@given(st.lists(st.integers()))" which if applied to test case function, generates some random list of int, which is then assigned to parameter of test case.
+For more information about hypothesis or in general about property based testing, check below links:
+- https://hypothesis.readthedocs.io/en/latest/quickstart.html
+- https://hypothesis.works/articles/what-is-property-based-testing/
+- http://blog.jessitron.com/2013/04/property-based-testing-what-is-it.html
+
 
 Running the test suite
 ----------------------