util/flash_jlink: Enforce python3
util/flash_jlink: Enforce python3

Python 3 is required for the subprocess.run line.
Enforcing python 3 helps our non-chroot devs.

BRANCH=none
BUG=none
TEST=# Outside chroot
     ./util/flash_jlink.py
     # Ensure the error does not reference subprocess.run

Signed-off-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Change-Id: I057c1b00696a4b356f162795fd1794eba9c54bd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2226877
Commit-Queue: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec