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

Hello !
I have the same issue on MacOSX Mojave 10.14.6. I updated my terminal profiles, as well as my $PATH to make sure the last version of Python is called :

bash-3.2$ python --version
Python 3.9.0
bash-3.2$ python
Python 3.9.0 (v3.9.0:9cf6752276, Oct  5 2020, 11:29:23)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import serial
>>> serial
<module 'serial' from '/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/serial/__init__.py'>
>>>

However, I still get the following message from Arduino IDE v1.8.13 :

Traceback (most recent call last):
  File "/Users/.../Library/Arduino15/packages/m5stack/tools/esptool_py/3.0.0/esptool.py", line 39, in <module>
    import serial
ImportError: No module named serial

I'm trying to use the new M5Paper so I cannot downgrade to 1.0.5, as the device is not referenced in it !

Thanks for your help and your amazing work on M5Paper
Lydéric