pw_env_setup: Remove newlines from python path

The python path extracted from pyvenv.cfg included a newline, causing
repeated bootstraps to fail because the python path didn't match what
was expected and the virtualenv was deleted.

Change-Id: Id69799611dbd42f8059be67452d6112be4100a3e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49302
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: David Rogers <davidrogers@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>