lang/python/python-package-install.sh: assign SOURCE_DATE_EPOCH to PYTHONHASHSEED

lang/python/python-package-install.sh: assign SOURCE_DATE_EPOCH to PYTHONHASHSEED

Following a discussion on bugs.python.org:
* https://bugs.python.org/issue29708
* https://bugs.python.org/msg313384

It seems that setting a fixed value to PYTHONHASHSEED guarantees that
the bytecodes are generated consistently/in a reproducible manner.

Hopefully, this is the last bit to make Python3 build reproducible.
Tested this locally on a few files [that were not reproducible without
this change].

The PYTHONHASHSEED is only assigned to the host Python/Python3 during
compilation of byte-codes [from python source].

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>