Hello,
It seems that Arduino IDE is force-using the MacOSX official python package and not any other version installed (even if $PATH or alias in profile force to use python3).
I succeed in solving this issue by locating the site-packages of the MacOSX python and forcing an installation of pyserial in this specified target.
Step1 - Locate site-packages target :
bash-3.2$ python
Python 2.7.16 (default, Jan 27 2020, 04:46:15)
[GCC 4.2.1 Compatible Apple LLVM 10.0.1 (clang-1001.0.37.14)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> print sys.path
[''..., '/Library/Python/2.7/site-packages',...']
>>>
Step2 - Install pyserial in the desired target
bash-3.2$ sudo pip3 install --target /Library/Python/2.7/site-packages pyserial
Step3 - verify correct installation
bash-3.2$ python
Python 2.7.16 (default, Jan 27 2020, 04:46:15)
[GCC 4.2.1 Compatible Apple LLVM 10.0.1 (clang-1001.0.37.14)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import serial
>>> serial
<module 'serial' from '/Library/Python/2.7/site-packages/serial/__init__.py'>
>>>
Everything worked on my side after that.
Hope it helps !
Lydéric