Fix f-string debug format parsing for Python 3.8+ ({expr=})

[aryanyk]

Summary

This PR fixes Issue #115, where Python 3.8+ f-string debug format ({expr=}) fails to parse correctly and raises:

AnnotationError: Expected '!' but found '='

The fix is scoped to Python 3.8–3.11, which are within pasta’s supported range.

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Background

Python 3.8 introduced debug f-strings:

f"{a=}"

Unlike regular f-strings, {expr=}:

• Leaves '=' in the token stream
• Implicitly sets conversion=repr
• Does not include an explicit '!' in the source

The current logic in visit_FormattedValue assumes that conversion must be introduced by '!', which causes parsing to fail.

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Root Cause

When parsing {expr=}:

1. The expression tokens are consumed
2. The parser expects '!' for conversion
3. '=' is encountered instead
4. An error is raised

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Fix

The fix updates visit_FormattedValue to:

1. Explicitly consume the debug specifier '='
2. Make conversion parsing conditional on the presence of '!'
3. Only parse the conversion character when '!' is present

This matches CPython’s behavior.

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Tests

Added test_fstring_debug_format covering:

• f"{a=}"
• f"{a=!r}"
• f"{a=:d}"
• f"{a=!r:10}"
• f"prefix {x=} suffix"

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Verification

Tests pass on:

• Python 3.10
• Python 3.11

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Notes

Python 3.12+ introduces PEP 701 (PEG-based f-string parsing), which is a separate compatibility issue and not addressed here.

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