Properly fix slotscheck for Python 3.12

[dantownsend]

When we upgraded to Python 3.12 slotscheck started failing because we inherit from Generic which no longer defines __slots__:

piccolo/piccolo/engine/base.py

Line 27 in 6cb005b

class Engine(t.Generic[TransactionClass], metaclass=ABCMeta):

In this PR #888 we stopped running slotscheck on Python 3.12 for now, as getting Python 3.12 support out was quite pressing.

piccolo/scripts/lint.sh

Lines 23 to 31 in 6cb005b

	echo "Running slotscheck..."
	# Currently doesn't work for Python 3.12 - so skipping until we can get a proper fix.
	pythonVersion=$(python -c 'import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}")')
	if [ "$pythonVersion" == '3.12' ]
	then
	echo "Skipping Python 3.12 for now"
	else
	python -m slotscheck $MODULES
	fi

Here's the corresponding issue on the slotscheck repo ariebovenberg/slotscheck#201