The python-3.6.5-macosx10.6.pkg installs pip3 as pip. This means if you have both python2.7.x and 3.6.x you get unexpected and undesirable behavior. According to the release notes in 3.6.5:

Python 3 and Python 2 Co-existence

Python.org Python 3.6 and 2.7.x versions can both be installed on your system and will not conflict. Command names for Python 3 contain a 3 in them, python3 (or python3.6), idle3 (or idle3.6), pip3 (or pip3.6), etc.  Python 2.7 command names contain a 2 or no digit: python2 (or python2.7 or python), idle2 (or idle2.7 or idle), etc.

The release notes for Python2.7.14 have a similarly worded note on Python 3 and Python 2 co-existence.

For both Pythons to properly coexist you must install Python2.7.x after installing Python3.6.x or manually fix the changes that the Python installers make to your ~/.profile PATH environmental variable.

$ which pip3
/Library/Frameworks/Python.framework/Versions/3.6/bin/pip3

$ ls -l $(dirname $(which pip3))
[SNIP]
-rwxr-xr-x  1 gilw  admin    263 Apr 16 13:05 pip
-rwxr-xr-x  1 gilw  admin    263 Apr 16 13:05 pip3
-rwxr-xr-x  1 gilw  admin    263 Apr 16 13:05 pip3.6
[SNIP]