CI: Quieter doctests
ref 
https://github.com/pandas-dev/pandas/pull/23215#issuecomment-431127737

cc @datapythonista @jbrockmendel 
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
``` # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23232?src=pr&el=h1) Report
> Merging [#23232](https://codecov.io/gh/pandas-dev/pandas/pull/23232?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b54e20b4c05727268857dc8cd8acd065f70c6048?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23232/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23232?src=pr&el=tree)

```diff
@@           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)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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