Add isspace API in str #5995

Job	Run time
LPython CI (3.10, ubuntu-latest)	13m 13s
LPython CI (3.10, macos-latest)	16m 39s
Check Debug build	2m 1s
LPython CI (3.10, windows-2019)	24m 3s
Build LPython to WASM	8m 56s
Test CPython Interop (@pythoncall)	2m 15s
Run Integration tests with Python 3.9	2m 25s
Check Release build	2m 21s
Run SymPy tests	2m 32s
Run Integration tests with Python 3.10	1m 56s
Test PIP Installable Packages	1m 48s
Run Integration tests with Python 3.11	2m 27s
Run Integration tests with Python 3.12	2m 1s
	1h 22m 37s

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