gdb: pyproject.toml: set pyright typeCheckingMode = "strict"

gdb: pyproject.toml: set pyright typeCheckingMode = "strict"

While working on other projects, I found the pyright type checker very
helpful when editing Python code.  I don't think I have to explain the
advantages of type checking to a crowd used to C/C++.

Setting typeCheckingMode to "strict" makes pyright flag a bit more type
issues than the default of "basic".

Change-Id: I38818ec59f7f73c2ab020cc9226286cdd485abc7
Reviewed-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>