style: fix PEP 8 violations in Python code

[yurekami]

Summary

This PR fixes Python code style issues to follow PEP 8 best practices.

Changes

1. Use assert not causal instead of assert causal == False (flash_mla_interface.py)

   • PEP 8 E712: comparison to False should be not expr
   • Also improved the error message clarity

2. Catch specific exceptions instead of bare Exception (setup.py)

   • Changed except Exception to except (subprocess.CalledProcessError, FileNotFoundError, OSError)
   • Added explanatory comment for the fallback behavior

Before/After

# Before
assert causal == False, "causal must be `false` if sparse attention is enabled."

# After  
assert not causal, "causal must be False when sparse attention is enabled (indices provided)"

# Before
except Exception as _:

# After
except (subprocess.CalledProcessError, FileNotFoundError, OSError):
    # Fallback to timestamp if git is not available or not in a git repo

Test plan

• Code passes Python syntax check
• No functional changes

🤖 Generated with Claude Code