[Eric] install.py does not seem to respect virtual environments

[Boylan, Ross]

I tried to do an install from the contents of the zip installation file for MS-Windows from within a virtual environment.  It appears to me that install.py does not respect the virtual environment, leading to failure.  I notice a number of existing bug reports (e.g., 239, perhaps 310), all closed as rejected, seem to fit the same pattern.  The rejection interpreted the problems as being with the setup of the  computer on which the problem was reported, but I believe they are the result of eric's install.py handling virtual environments inconsistently, mostly by ignoring them.

The machine where this happened can't send email, but here are some highlights.  My setup was a bit unusual.  Windows 10 home.
I followed these steps for setup:
1. install Python 3.8 64 bit for myself only, except that the py command was installed for everyone.  Told it to change my PATH to find this python.
2. Install Python 3.7 64 bit mostly the same way, except skipped installing the py command at all and did not add it to my PATH. (If you're curious: I found PySide2 was not yet available for 3.8).
3. py -3.7 -m venv BTF to create a virtual environment.
4. In an existing command prompt (so I'm pretty sure it did not have the PATH update from step 1) cd into BTF and activate.

Some of the evidence of a problem.
1. Although py --version returns 3.7 inside that environment, py install.py in the appropriate directory from the zip file prints 
Checking Dependencies
Python Version: 3.8.0
2. It also reports not finding packages that I had installed inside the venv.
3. When I let if fetch some of the packages, the messages indicated they are being installed into 3.8.
4. And when it later tries to use those packages, e.g., PyQt5 it reports it can't find them, presumably because it is looking inside the venv (and the complain goes away if I manually install it in the venv).

After a few rounds of doing manual installs of dependent packages in the venv I gave up and ran
pip install eric-ide
and that seems to have worked: everything necessary is in the venv and I can launch the GUI from within the venv.  Of course, I had to re-download everything that was in the zip file again.
Ross