When Python is installed on Windows separately (i.e. Windows Store,
Python.org MSI, etc.) it set the PYTHONHOME environment variable to
the installed path.

When we call our own portable Python, it tries to use the support
PYC files in PYTHONHOME.  If they're from a different version of
Python, however, they won't work.  Even though we're running our
own distributed Python.exe, we are crashing on the system-wide
Python installation.

Add -I to all python3 calls, ignore PYTHONHOME/etc. environment vars.

Tested under Ubuntu 18.04 Linux w/o any ill effects (we ship pyserial
ourselves and add it manually to the Python search path).

Fixes esp8266#8096, or should as I understand it.