Fix obscure failures of datetime-related tests due to the datetime tests failing to restore the system state completely after testing the pure-Python versions.

Remove setting hash seed to regrtest's random seed and re-execv()ing: this doesn't preserve Python flags and fails from a temp directory.

Fix "sys.path modified" warning in test_strlit, by not replacing sys.path itself, only its contents.

Merge from 3.1: Issue #13703: add a way to randomize the hash values of basic types (str, bytes, datetime)
in order to make algorithmic complexity attacks on (e.g.) web apps much more complicated.

The environment variable PYTHONHASHSEED and the new command line flag -R control this
behavior.

in order to make algorithmic complexity attacks on (e.g.) web apps much more complicated.

The environment variable PYTHONHASHSEED and the new command line flag -R control this
behavior.

Also wrap two looong lines.

upon malformed POST request.

upon malformed POST request.

to to makes bytes mutable, before deciding to add bytearrays.)

Closes #11689.

Closes #13995.

Initial patch by Johannes Vogel.