CI: Quieter doctests

[TomAugspurger]

ref
#23215 (comment)

cc @datapythonista @jbrockmendel

[TomAugspurger]

Previously, lint failures were being pushed far away from the exit code due to a new line per doctest.

If there is a failure, it'll still be printed out.

inside ./ci/code_checks.sh
Doctests frame.py
.......................F...........................                                                                                                  [100%]
========================================================================= FAILURES =========================================================================
________________________________________________________ [doctest] pandas.core.frame.DataFrame.info ________________________________________________________
2199         DataFrame.memory_usage: Memory usage of DataFrame columns.
2200
2201         Examples
2202         --------
2203         >>> int_values = [1, 2, 3, 4, 5]
2204         >>> text_values = ['alpha', 'beta', 'gamma', 'delta', 'epsilon']
2205         >>> float_values = [0.0, 0.25, 0.5, 0.75, 1.0]
2206         >>> df = pd.DataFrame({"int_col": int_values, "text_col": text_values,
2207         ...                   "float_col": float_values})
2208         >>> df
Differences (unified diff with -expected +actual):
    @@ -1,3 +1,3 @@
    -   int_col text_col  float_co
    +   int_col text_col  float_col
     0        1    alpha       0.00
     1        2     beta       0.25

/Users/taugspurger/sandbox/pandas/pandas/core/frame.py:2208: DocTestFailure

[jorisvandenbossche]

I used that originally locally to see which ones were running/failing, but indeed on travis it is mainly important to see the error

[codecov]

Codecov Report

Merging #23232 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #23232   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50950    50950           
=======================================
  Hits        46975    46975           
  Misses       3975     3975

Flag	Coverage Δ
#multiple	90.62% <ø> (ø)	⬆️
#single	42.27% <ø> (ø)	⬆️

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