Revert FrozenList changes (doc build slowdown, #15559)
See #15559. This temporarily reverts #15506, to see if this fixes the doc build slowdown.
So I am totally puzzled about why, but reverting this *does* fix the doc build timeout, see https://travis-ci.org/pandas-dev/pandas/builds/207655451

While we figure this out, I would like to merge this revert for now, because with current master it makes doc building really unbearable (certainly with the docathon coming up). sure pls create an issue (or leave this one open and i'll take a look)

where was the doc build hanging (what part)? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15566?src=pr&el=h1) Report
> Merging [#15566](https://codecov.io/gh/pandas-dev/pandas/pull/15566?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7ae4fd10b1581d77b4cfbf594e3b444af7456876?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15566      +/-   ##
==========================================
- Coverage   91.07%   91.04%   -0.03%     
==========================================
  Files         136      136              
  Lines       49167    49155      -12     
==========================================
- Hits        44777    44754      -23     
- Misses       4390     4401      +11
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15566?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.48% <ø> (ø)` | :white_check_mark: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.22% <100%> (ø)` | :white_check_mark: |
| [pandas/core/reshape.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS5weQ==) | `99.25% <100%> (ø)` | :white_check_mark: |
| [pandas/indexes/frozen.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvZnJvemVuLnB5) | `93.54% <100%> (+0.3%)` | :white_check_mark: |
| [pandas/tools/concat.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL2NvbmNhdC5weQ==) | `97.62% <100%> (ø)` | :white_check_mark: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.78% <0%> (-0.35%)` | :x: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.87% <0%> (-0.19%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.83% <0%> (-0.1%)` | :x: |
| [pandas/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvYmFzZS5weQ==) | `96.22% <0%> (+0.05%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15566?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15566?src=pr&el=footer). Last update [7ae4fd1...befd858](https://codecov.io/gh/pandas-dev/pandas/compare/7ae4fd10b1581d77b4cfbf594e3b444af7456876...befd85804009b45c7273dbc7d2b32b8de64853f5?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). The full 'reading' part (that is AFAIK the part where the ipython code blocks are executed). I am thinking of how we can try to profile that. using this to debug (it prints a LOT)
``-vvv`` is verbose, ``-N`` removes the color output, and removed the ``-P`` to try

```
diff --git a/doc/make.py b/doc/make.py
index 8a6d4e5..3c47409 100755
--- a/doc/make.py
+++ b/doc/make.py
@@ -197,7 +197,7 @@ def html():
                 print(e)
                 print("Failed to convert %s" % nb)
 
-    if os.system('sphinx-build -P -b html -d build/doctrees '
+    if os.system('sphinx-build -vvv -b html -d build/doctrees '
                  'source build/html'):
         raise SystemExit("Building HTML failed.")
```
we should prob *always* add ``-v`` just to have it print out files as it reads them.

still not sure why it fails
 so going to merge this and we can figure out later.