move decode_byte_string out of tests

[dboures]

Fixes issue with importing test token caused by having decode_byte_string in the tests module

"
When I try to import the Token type, it fails with:

from spl.token.client import Token as SplToken

File "/usr/local/lib/python3.9/site-packages/spl/token/client.py", line 14, in
from spl.token.core import AccountInfo, MintInfo, _TokenCore
File "/usr/local/lib/python3.9/site-packages/spl/token/core.py", line 18, in
from tests.integration.utils import decode_byte_string
ModuleNotFoundError: No module named 'tests.integration'
"

[codecov]

Codecov Report

Merging #91 (8496e4c) into master (5347a7b) will decrease coverage by 0.09%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   88.30%   88.21%   -0.10%     
==========================================
  Files          32       32              
  Lines        2121     2130       +9     
==========================================
+ Hits         1873     1879       +6     
- Misses        248      251       +3