DOC: Remove preference for pytest paradigm in assert_raises_regex
`tm.assert_raises_regex` is a more compact version of the following:

~~~python
with pytest.raises(<exc_type>) as exc_info:
   f(...)
exc_info.match(<regex>)
~~~

No reason for the docs to say that we prefer the `pytest` paradigm over our own.

xref <a href="https://github.com/pandas-dev/pandas/pull/16511#discussion_r118771331">#16511 (comment)</a>