signature: increase compatibility across Python versions

signature: increase compatibility across Python versions

Underscores in number literals are a recent Python feature which only
was introduced in version 3.6. The sigrok project claims compatibility
with previous Python versions, but the signature decoder fails to load
with this error:

  $ pulseview
  srd: SyntaxError: Failed to load decoder signature: import by name failed: invalid syntax (pd.py, line 138)
  srd: Traceback (most recent call last):
    File "/home/user/share/libsigrokdecode/decoders/signature/__init__.py", line 25, in <module>
      from .pd import Decoder
    File "/home/user/share/libsigrokdecode/decoders/signature/pd.py", line 138
      incoming = (bin(shiftreg & 0b0000_0010_1001_0001).count('1') + data) & 1
                                                     ^
  SyntaxError: invalid syntax

Use the more compact hex presentation for a magic binary pattern. This
obsoletes the necessity to separate groups of bits for readability.