DOC: Remove preference for pytest paradigm in assert_raises_regex

[gfyoung]

tm.assert_raises_regex is a more compact version of the following:

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 #16511 (comment)

[codecov]

Codecov Report

Merging #16518 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #16518   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         161      161           
  Lines       51063    51063           
=======================================
  Hits        46365    46365           
  Misses       4698     4698

Flag	Coverage Δ
#multiple	88.63% <ø> (ø)	⬆️
#single	40.15% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	80.98% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6649157...a6ccc38. Read the comment docs.

[codecov]

Codecov Report

Merging #16518 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #16518   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         161      161           
  Lines       51063    51063           
=======================================
  Hits        46365    46365           
  Misses       4698     4698

Flag	Coverage Δ
#multiple	88.63% <ø> (ø)	⬆️
#single	40.15% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	80.98% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6649157...a6ccc38. Read the comment docs.