[BUG][BLD] revert DEF component of #21878
#21878 introduced a subtle build problem that is not caught by the CI.  Running `python setup.py build_ext --inplace` followed by `python3 setup.py build_ext --inplace` causes compile-time errors in py3 (or if running these in the opposite order, errors in py2).  These are fixed by running `python setup.py clean` in between.

This reverts the relevant changes.

For anyone curious: cython supports syntax:

```
DEF foo = [...]

IF foo:
    [...]
ELSE:
    [...]
```

and these IF/ELSE conditions get evaluated at compile-time.  #21878 incorrectly assumed that "compile-time" in this context meant ".c -> .so" time, not ".pyx -> .c" time.  In this we used:

```
DEF PY2 = str == bytes

IF PY2:
    [...]
ELSE:
    [...]
```

so after running `setup.py build_ext --inplace` in py2, the ".c" file we end up with has already gotten rid of the PY3 branches.  When we run `python3 setup.py build_ext --inplace` cython uses the existing .c file, tries to compile it to .so, and breaks.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21919?src=pr&el=h1) Report
> Merging [#21919](https://codecov.io/gh/pandas-dev/pandas/pull/21919?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/63eff04af6f343557b30e656e94d76030e039029?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/21919/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21919?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21919   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <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/21919?src=pr&el=footer). Last update [63eff04...d939bde](https://codecov.io/gh/pandas-dev/pandas/pull/21919?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback @WillAyd quick take before I merge? lgtm

yeah ideally these would be compile time checks 