.mbedignore - add venv folder

[JanneKiiskila]

This will create some nasty surprises in case you do a "proper clean installation of Python 2" for mbed cli (in case you want to run Mbed OS 5.15.x). I.e.

virtualenv venv -p /usr/bin/python2.7
pip install mbed-cli
pip install mbed-os/requirements.txt

It will try to compile EVERYTHING - as per usual and then fails when it does not find various Python compilation things.

Compile [ 97.3%]: _speedups.c
[Error] pyconfig.h@122,3: #error unknown multiarch location for pyconfig.h
[Error] pyconfig.h@122,3: #error unknown multiarch location for pyconfig.h
[Error] pyconfig.h@122,3: #error unknown multiarch location for pyconfig.h
[Error] _speedups.c@18,8: unknown type name 'Py_UNICODE'
1:32

Took a good while for me to figure it out... :-(

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