Update Installing Packages to reflect deprecation of pyvenv

[stevepiercy]

Per https://hg.python.org/cpython/rev/6ab285820db2#l1.8 pyvenv will be deprecated in 3.6 in favor of python3 -m venv. Suggest that installing.rst be reviewed and updated accordingly.

Include citations or references to reasoning for the change, perhaps in footnotes.

[stevepiercy]

See also:
https://bugs.python.org/issue27285