Just a sanity check ... I can't reproduce the issue with the latest versions of Python (I haven't made any changes to the venv code):

On Python 3.5:

 vinay@ubuntu:~/projects/python/3.5$ ./python
Python 3.5.3rc1+ (3.5:d9a64d00a439, Jan 13 2017, 10:26:23) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
 vinay@ubuntu:~/projects/python/3.5$ ./python -m venv --system-site-packages /tmp/venv35
 vinay@ubuntu:~/projects/python/3.5$ /tmp/venv35/bin/pip --version
pip 9.0.1 from /tmp/venv35/lib/python3.5/site-packages (python 3.5)

Now try on 3.6:

 vinay@ubuntu:~/projects/python/3.5$ cd ../3.6
 vinay@ubuntu:~/projects/python/3.6$ ./python
Python 3.6.0+ (3.6:17ca7c4dd583, Jan 13 2017, 10:24:18) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
 vinay@ubuntu:~/projects/python/3.6$ ./python -m venv --system-site-packages /tmp/venv36
 vinay@ubuntu:~/projects/python/3.6$ /tmp/venv36/bin/pip --version
pip 9.0.1 from /tmp/venv36/lib/python3.6/site-packages (python 3.6)

Now try on default (to-be-3.7):

 vinay@ubuntu:~/projects/python/default$ cd ../default
 vinay@ubuntu:~/projects/python/default$ ./python
Python 3.7.0a0 (default:0503024831ad, Jan 13 2017, 10:22:09) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
 vinay@ubuntu:~/projects/python/default$ ./python -m venv --system-site-packages /tmp/venv37
 vinay@ubuntu:~/projects/python/default$ /tmp/venv37/bin/pip --version
pip 9.0.1 from /tmp/venv37/lib/python3.7/site-packages (python 3.7)

Let's try installing a package: pick a small, one-file one.

 vinay@ubuntu:~/projects/python/default$ /tmp/venv37/bin/pip install python-gnupg
Collecting python-gnupg
  Downloading python_gnupg-0.3.9-py2.py3-none-any.whl
Installing collected packages: python-gnupg
  Installing from a newer Wheel-Version (1.1)
Successfully installed python-gnupg-0.3.9
 vinay@ubuntu:~/projects/python/default$ ls -l /tmp/venv37/lib/python3.7/site-packages/
total 84
-rw-rw-r--  1 vinay vinay   126 Jan 13 11:00 easy_install.py
-rw-rw-r--  1 vinay vinay 51019 Jan 13 11:01 gnupg.py
drwxrwxr-x 11 vinay vinay  4096 Jan 13 11:00 pip
drwxrwxr-x  2 vinay vinay  4096 Jan 13 11:00 pip-9.0.1.dist-info
drwxrwxr-x  5 vinay vinay  4096 Jan 13 11:00 pkg_resources
drwxrwxr-x  2 vinay vinay  4096 Jan 13 11:01 __pycache__
drwxrwxr-x  2 vinay vinay  4096 Jan 13 11:01 python_gnupg-0.3.9.dist-info
drwxrwxr-x  5 vinay vinay  4096 Jan 13 11:00 setuptools
drwxrwxr-x  2 vinay vinay  4096 Jan 13 11:00 setuptools-28.8.0.dist-info

As you can see, gnupg.py is in the list, showing that the package was correctly installed in the virtualenv.