Add MSB validation to raise when timestamp overflows 128 bits

[ahawker]

Status: Ready

If merged, this PR adds validation checks to raise ValueError when decoding ULID values that will overflow 128 bits.

Fixes #468

>>> import ulid
>>> ulid.parse('7ZZZZZZZZZZZZZZZZZZZZZZZZZ')
<ULID('7ZZZZZZZZZZZZZZZZZZZZZZZZZ')>
>>> ulid.parse('8ZZZZZZZZZZZZZZZZZZZZZZZZZ')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/hawker/src/github.com/ahawker/ulid/ulid/api.py", line 85, in parse
    return from_str(value)
  File "/Users/hawker/src/github.com/ahawker/ulid/ulid/api.py", line 190, in from_str
    return ulid.ULID(base32.decode_ulid(value))
  File "/Users/hawker/src/github.com/ahawker/ulid/ulid/base32.py", line 251, in decode_ulid
    encoded = str_to_bytes(value, 26)
  File "/Users/hawker/src/github.com/ahawker/ulid/ulid/base32.py", line 373, in str_to_bytes
    raise ValueError('Timestamp value too large and will overflow 128-bits. Must be between b"0" and b"7"')
ValueError: Timestamp value too large and will overflow 128-bits. Must be between b"0" and b"7"

[codecov[bot]]

Codecov Report

Merging #471 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #471   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          364       368    +4     
  Branches        75        77    +2     
=========================================
+ Hits           364       368    +4     

Impacted Files	Coverage Δ
ulid/base32.py	100.00% <100.00%> (ø)

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