FIX: const-correctness in numpy helpers
In python 3.7 the return type of PyUnicode_AsUTF8 changed
from (char *) to (const char *).

PyUnicode_FromString also takes (const char *) as input, also be
explicit about that.

https://bugs.python.org/issue28769
commit 2a404b63d48d73bbaa007d89efb7a01048475acd in cpython

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks @tacaswell 

haven't touched 3.7 yet!!! ping on green. @tcaswell do you have a 3.7 going on CI? my day-to-day enviroment at home is cpython master + master branch of most of the scipy stack :smiling_imp: . # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19749?src=pr&el=h1) Report
> Merging [#19749](https://codecov.io/gh/pandas-dev/pandas/pull/19749?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c086a518fe728deda168a4f800e4224e497ad19a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19749/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19749?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19749      +/-   ##
==========================================
- Coverage    91.6%    91.6%   -0.01%     
==========================================
  Files         150      150              
  Lines       48864    48864              
==========================================
- Hits        44763    44761       -2     
- Misses       4101     4103       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19749?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19749/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19749?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/19749?src=pr&el=footer). Last update [c086a51...f97a40c](https://codecov.io/gh/pandas-dev/pandas/pull/19749?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @tacaswell happy to have these