Stringify the cast type, for <3.10 support

[tusharsadhwani]

Since the types here are an argument, we need to manually stringify them to avoid causing runtime errors.

[codecov-commenter]

Codecov Report

Merging #351 (54b72f5) into css (386aa6d) will increase coverage by 3.63%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##              css     #351      +/-   ##
==========================================
+ Coverage   38.06%   41.70%   +3.63%     
==========================================
  Files          85       86       +1     
  Lines        6526     7014     +488     
==========================================
+ Hits         2484     2925     +441     
- Misses       4042     4089      +47     

Flag	Coverage Δ
unittests	41.70% <40.00%> (+3.63%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/textual/_event_broker.py	0.00% <ø> (ø)
src/textual/_timer.py	50.00% <ø> (+17.14%)	⬆️
src/textual/_types.py	78.94% <ø> (ø)
src/textual/app.py	0.00% <0.00%> (ø)
src/textual/css/tokenizer.py	89.01% <ø> (-0.12%)	⬇️
src/textual/drivers/linux_driver.py	0.00% <0.00%> (ø)
src/textual/layouts/grid.py	21.42% <ø> (-8.04%)	⬇️
src/textual/renderables/blank.py	0.00% <0.00%> (ø)
src/textual/scrollbar.py	0.00% <0.00%> (ø)
src/textual/views/_dock_view.py	0.00% <0.00%> (ø)
... and 56 more

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[willmcgugan]

Thanks