Source venv when testing and tweak py shebang
Source venv when testing and tweak py shebang

For each python related target, source the venv before running the
python interpreter.

Introduce PYTHON_SHEBANG which is used to force a specific shebang,
while PYTHON is used where a reference to the python binary path is
required.

This change makes it possible to use /usr/bin/env python as a shebang
while still preserving the old semantics for programs that make
assumptions about PYTHON (pyrest_temporary_server.cc)

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