Trying to pip install with "examples" optional deps fails

Running `pip install traitsui[examples]` fails with
```
$ pip install traitsui[examples]
Collecting traitsui[examples]
Using cached traitsui-7.1.1.tar.gz (5.8 MB)
Processing /Users/aayres/Library/Caches/pip/wheels/25/73/54/c034c1b79c3bcac3102f82236ee7fe05e341cd9839ae94c07c/traits-6.1.1-cp36-cp36m-macosx_10_9_x86_64.whl
Processing /Users/aayres/Library/Caches/pip/wheels/8a/cb/ce/962f0e0badb2541174d54446cd2c42be4566752b1cd9f9a335/pyface-7.2.0-py3-none-any.whl
Collecting apptools
Using cached apptools-5.0.0.tar.gz (182 kB)
Collecting chaco
Using cached chaco-4.8.0.tar.gz (887 kB)
ERROR: Command errored out with exit status 1:
command: /Users/aayres/.edm/envs/test/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/j9/ps8s26f91_1_lgf7mtd4smyr0000gn/T/pip-install-745yd40j/chaco/setup.py'"'"'; __file__='"'"'/private/var/folders/j9/ps8s26f91_1_lgf7mtd4smyr0000gn/T/pip-install-745yd40j/chaco/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/j9/ps8s26f91_1_lgf7mtd4smyr0000gn/T/pip-install-745yd40j/chaco/pip-egg-info
cwd: /private/var/folders/j9/ps8s26f91_1_lgf7mtd4smyr0000gn/T/pip-install-745yd40j/chaco/
Complete output (5 lines):
Traceback (most recent call last):
File "", line 1, in
File "/private/var/folders/j9/ps8s26f91_1_lgf7mtd4smyr0000gn/T/pip-install-745yd40j/chaco/setup.py", line 7, in
from numpy import get_include
ModuleNotFoundError: No module named 'numpy'
----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
```

numpy is needed in the environment already in order to install chaco (more specifically to install kiva)