BLD: bump numpy min version to 1.15.4
- [x] closes #33718
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Mothballing to clear the queue @WillAyd the MacOSPy36 failure is at build-time, any thoughts?

@TomAugspurger the LinuxPY36 failure looks like its a conda-dependency issue, any thoughts?

One of the other two failures should be fixed by #33898 leaving one more that i need to troubleshoot. Seems to be an issue between NumPy 1.15.4 and python=3.6.1

```
 ~  conda create -n ttttt -c defaults -c conda-forge numpy=1.15.4 python=3.6.1                                                                                                                                                                                                                                                           10.9s  Thu Apr 30 16:20:32 2020
Collecting package metadata (current_repodata.json): done
Solving environment: failed with repodata from current_repodata.json, will retry with next repodata source.
Collecting package metadata (repodata.json): done
Solving environment: |
Found conflicts! Looking for incompatible packages.
This can take several minutes.  Press CTRL-C to abort.
failed

UnsatisfiableError: The following specifications were found to be incompatible with each other:

Output in format: Requested package -> Available versions

Package python conflicts for:
python=3.6.1
numpy=1.15.4 -> python[version='>=2.7,<2.8.0a0|>=3.6,<3.7.0a0|>=3.7,<3.8.0a0']
numpy=1.15.4 -> mkl_fft[version='>=1.0.6,<2.0a0'] -> python[version='2.7.*|3.5.*|3.6.*|>=3.5,<3.6.0a0|>=3.8,<3.9.0a0']
Note that strict channel priority may have removed packages required for satisfiability.
```

Python 3.6.2 seems fine. I'd recommend removing the `defaults` channel from that build. Haven't debugged macOS failure but one thing worth updating first perhaps is the dependency file for it. I think it still has numpy 1.14 pinned

https://github.com/pandas-dev/pandas/blob/1940fcea056325cbe878c72078c51f80ea8482f4/ci/deps/azure-macos-36.yaml#L22 > Python 3.6.2 seems fine. I'd recommend removing the defaults channel from that build.

first couple tries didnt work, any other suggestions?  no rush If you're able to reproduce locally, then binary search on the env file is often best (comment out half the file, `conda env create -f ...`, see if it breaks).

Turns out scipy=0.19.0 was the (or at least a) problem. It's not compatible with NumPy 1.15.4.

I'd recommend bumping scipy to 1.2.0, which is the first release after NumPy 1.15.4. Can you bump the versions in `pandas/compat/_optional.py` for scipy and any other optional deps that are increased? > I'd recommend bumping scipy to 1.2.0, which is the first release after NumPy 1.15.4.

that did the trick, thanks great!