[pgAdmin4] [PATCH] Simplify Server's python setup

Hi,

in the process of packaging pgAdmin4 in Arch Linux I ran into trouble
getting it to start, which I reduced to a bad sys.path.

The attached patch simplifies the Python initialization. If the program
name is set to match the venv's interpreter executable, Py_Initialize will
set up sys.path to match the virtualenv.

This means that, assuming the venv is in the right directory, no further
configuration of the PythonPath should be necessary.

I tested this using Python 2.7.13 and Python 3.6. I could not test this on
Windows or OSX, but I made an effort at supporting them. Since this patch
offloads all the path gathering to Python itself, the differences between
the platforms are minimal.

If relevant, the script I use to package is at
https://pkgbuild.com/~heftig/packages/pgadmin4/PKGBUILD . The interesting
stuff is in the build() and package() functions, which should be
self-explanatory.

Thanks,
Jan Steffens