BLD: Fixed pip install with no numpy
Closes #25193 

Still testing this locally and on https://github.com/pandas-dev/pandas-ci/pull/6
So a `pip install --no-binary :all: pandas-0.25.0.dev0+203.g3e652ac56.tar.gz` into an sdist I built with this PR works... Anything else we should verify? In https://github.com/pandas-dev/pandas-ci/pull/6 the CI run https://travis-ci.org/pandas-dev/pandas-ci/jobs/502590460 failed with

```
[pip-test-env before]
# packages in environment at /home/travis/miniconda3/envs/pip-test-env:
#
# Name                    Version                   Build  Channel
ca-certificates           2019.1.23                     0  
certifi                   2018.11.29               py37_0  
libedit                   3.1.20181209         hc058e9b_0  
libffi                    3.2.1                hd88cf55_4  
libgcc-ng                 8.2.0                hdf63c60_1  
libstdcxx-ng              8.2.0                hdf63c60_1  
ncurses                   6.1                  he6710b0_1  
openssl                   1.1.1b               h7b6447c_0  
pip                       19.0.3                   py37_0  
python                    3.7.2                h0371630_0  
readline                  7.0                  h7b6447c_5  
setuptools                40.8.0                   py37_0  
sqlite                    3.26.0               h7b6447c_0  
tk                        8.6.8                hbc83047_0  
wheel                     0.33.1                   py37_0  
xz                        5.2.4                h14c3975_4  
zlib                      1.2.11               h7b6447c_3  
[pip install]
Processing ./dist/pandas-0.25.0.dev0+203.g3e652ac56.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "/home/travis/miniconda3/envs/pip-test-env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 359, in get_provider
        module = sys.modules[moduleOrReq]
    KeyError: 'numpy'
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-req-build-ngo5yevd/setup.py", line 738, in <module>
        ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
      File "/tmp/pip-req-build-ngo5yevd/setup.py", line 481, in maybe_cythonize
        numpy_incl = pkg_resources.resource_filename('numpy', 'core/include')
      File "/home/travis/miniconda3/envs/pip-test-env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 1144, in resource_filename
        return get_provider(package_or_requirement).get_resource_filename(
      File "/home/travis/miniconda3/envs/pip-test-env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 361, in get_provider
        __import__(moduleOrReq)
    ModuleNotFoundError: No module named 'numpy'
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-req-build-ngo5yevd/
real	0m1.582s
user	0m1.448s
sys	0m0.124s
The command "ci/install.sh" failed and exited with 1 during .
```

Which is good (that was against master).

I'm going to submit a temporary commit there that applies this patch.   # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=h1) Report
> Merging [#25568](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3e652ac565e054b763ccbe23006c02704f6cd1b1?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/25568/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25568   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         173      173           
  Lines       52966    52966           
=======================================
  Hits        48337    48337           
  Misses       4629     4629
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25568?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/25568?src=pr&el=footer). Last update [3e652ac...3d0a793](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=h1) Report
> Merging [#25568](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3e652ac565e054b763ccbe23006c02704f6cd1b1?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25568/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25568      +/-   ##
==========================================
+ Coverage   91.26%   91.26%   +<.01%     
==========================================
  Files         173      173              
  Lines       52966    52966              
==========================================
+ Hits        48337    48338       +1     
+ Misses       4629     4628       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25568/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25568?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/25568?src=pr&el=footer). Last update [3e652ac...e2a3272](https://codecov.io/gh/pandas-dev/pandas/pull/25568?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. release note? thanks @TomAugspurger 