BUG: _cython_table bug fix
#21224 introduced a bug, where the ``conftest._cython_table`` isn't sorted by the function's name, but by the functions class name (i.e. "function"). This can make the sorting indeterministic and make the build fail on Python 2.7 and 3.5 at times.

This PR makes the sort order follow the functions names again and therefore be deterministic.
lgtm ping on green  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22110?src=pr&el=h1) Report
> Merging [#22110](https://codecov.io/gh/pandas-dev/pandas/pull/22110?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0b7a08b70444056f964bb636a74ac41ca0058d37?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/22110/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22110?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22110   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         170      170           
  Lines       50705    50705           
=======================================
  Hits        46680    46680           
  Misses       4025     4025
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22110?src=pr&el=footer). Last update [0b7a08b...0cddc82](https://codecov.io/gh/pandas-dev/pandas/pull/22110?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 All green.