New mypy error with asyncio.Task

[alexrudd2]

❯ git checkout 67d875b785d4f79da7b76413ae62ad71a1259fc3
Note: switching to '67d875b785d4f79da7b76413ae62ad71a1259fc3'.
❯ python3.11 -m mypy pymodbus
Success: no issues found in 58 source files
❯ git checkout dev
Previous HEAD position was 67d875b7 Solve serial close raise problem.
Switched to branch 'dev'
Your branch is up to date with 'upstream/dev'.

❯ python3.11 -m mypy pymodbus
pymodbus/client/base.py:267: error: Statement is unreachable  [unreachable]
pymodbus/client/base.py:274: error: Incompatible types in assignment (expression has type "Task[Any]", variable has type "None")  [assignment]
Found 2 errors in 1 file (checked 60 source files)

❯ python3.8 -m mypy pymodbus
Success: no issues found in 60 source files

It appears that Python3.11 mypy catches this, but Python3.8 does not (which is why the CI didn't flag it). My guess is that asyncio has improved its type definitions. Anyways, it's an easy fix with a hint; I'll prepare a PR.