Force pyserial version 3.5+ because 3.4 and earlier versions fail on
Big Sur with Python 3.8.

pyserial bug: https://github.com/pyserial/pyserial/issues/509
Bug tracking Python 3.9 support in LUCI/CIPD: crbug.com/1176014

Change-Id: I87a057e668394329655a8e75e1e17c98182f3a14
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/32126
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Mark Rawling <mwr@google.com>
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>