Fix regex-based version extraction in requirements.txt parsing

[Copilot]

requirements_version() failed to parse pretext lines with extras (e.g., pretext[prefigure] == 2.36.0) and contained a syntax error (gm flags appended to a string literal).

Changes

• pretext/utils.py

  • Remove duplicate import re
  • Fix SyntaxError: remove invalid gm suffix from regex string literal
  • Tighten version capture group from [\d\.]* → [\d\.]+ (require at least one digit; prevents matching pretext ==)
  • Store re.match() result to avoid double evaluation

• tests/test_utils.py

  • Add test_requirements_version() covering:
    • pretext, pretextbook, extras (pretext[prefigure], pretext[prefigure,extra])
    • Whitespace variations
    • Non-matching lines (numpy == ..., pretext ==, bare pretext)

# All of these now parse correctly
"pretext == 2.36.0"           # → "2.36.0"
"pretextbook == 1.2.3"        # → "1.2.3"
"pretext[prefigure] == 2.36.0"  # → "2.36.0"  (previously broken)

# These correctly return None
"pretext =="   # empty version
"numpy == 1.0.0"

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