DeprecationWarning in pp file saving

[acchamber]

🐛 Bug Report

When you run the Iris pytest, the following warning appears

lib/iris/tests/system_test.py: 1 warning
lib/iris/tests/test_cdm.py: 1 warning
lib/iris/tests/test_cube_to_pp.py: 58 warnings
lib/iris/tests/test_file_save.py: 8 warnings
lib/iris/tests/test_pp_cf.py: 6 warnings
lib/iris/tests/integration/test_pp.py: 2 warnings
lib/iris/tests/integration/fast_load/test_fast_load.py: 188 warnings
lib/iris/tests/unit/fileformats/um/fast_load/test_FieldCollation.py: 16 warnings
  /net/home/h05/achamber/git/iris/lib/iris/fileformats/pp.py:1199: DeprecationWarning: NumPy will stop allowing conversion of out-of-bound Python integers to integer arrays.  The conversion of -99 to uint32 will fail in the future.
  For the old behavior, usually:
      np.array(value).astype(dtype)
  will give the desired result (the cast overflows).
    lb[index] = header_elem

Simply replacing the offending line with `lb[index] = np.array(header_elem).astype(dtype) causes new errors, and I'm not familiar enough with the PP saving functions to be aware what is and isn't a problem. Reporting this so it's recorded for future viewing by someone with better understanding of the PP format.