signed8 and signed16 overflow U failures

[davidbenncsiro]

For some Python versions greater than 3.7 (e.g. 3.11), these UT errors occur:

FAILED tests/hydroscat/test_extract_raw.py::test_extract_D - OverflowError: Python integer 64292 out of bounds for int16
FAILED tests/hydroscat/test_extract_raw.py::test_extract_raw_with_H - OverflowError: Python integer 255 out of bounds for int8
FAILED tests/hydroscat/test_extract_raw.py::test_extract_H - OverflowError: Python integer 255 out of bounds for int8
FAILED tests/hydroscat/test_hydroscat.py::test_rawline2datadict_with_H - OverflowError: Python integer 255 out of bounds for int8