debian/rules: Set PYTHON=/usr/bin/python
debian/rules: Set PYTHON=/usr/bin/python

Override upstream's default shebang line in the generated svtplay-dl executable,
in accorance with Debian's Python policy (2.4.2).

Reference: https://www.debian.org/doc/packaging-manuals/python-policy/ch-python.html#s-interpreter_loc

olof/debian-svtplay-dl,olof/debian-svtplay-dl