gh-130664: support '_' (just as ',') in Decimal's formatting

```pycon
>>> from _decimal import Decimal as D
>>> format(D(1234567), '_')
'1_234_567'
>>> format(D(1234567), '020_')
'0_000_000_001_234_567'
>>> format(D('1234.56'), '07_')
'1_234.56'
>>> format(D('1.23456789'), '_')
'1.23456789'
>>> format(D('123.456789'), '_%')
'12_345.6789%'
>>> from _pydecimal import Decimal as D
>>> format(D(1234567), '_')
'1_234_567'
>>> format(D(1234567), '020_')
'0_000_000_001_234_567'
>>> format(D('1234.56'), '07_')
'1_234.56'
>>> format(D('1.23456789'), '_')
'1.23456789'
>>> format(D('123.456789'), '_%')
'12_345.6789%'
```

Author: skirpichev

Lib/_pydecimal.py

@@ -6098,7 +6098,7 @@ def _convert_for_comparison(self, other, equality_op=False):
 (?P<alt>\#)?
 (?P<zeropad>0)?
 (?P<minimumwidth>(?!0)\d+)?
-(?P<thousands_sep>,)?
+(?P<thousands_sep>[,_])?
 (?:\.(?P<precision>0|(?!0)\d+))?
 (?P<type>[eEfFgGn%])?
 \Z

Lib/test/test_decimal.py

@@ -1082,6 +1082,11 @@ def test_formatting(self):
             (',%', '123.456789', '12,345.6789%'),
             (',e', '123456', '1.23456e+5'),
             (',E', '123456', '1.23456E+5'),
+            # ... with '_' instead
+            ('_', '1234567', '1_234_567'),
+            ('07_', '1234.56', '1_234.56'),
+            ('_', '1.23456789', '1.23456789'),
+            ('_%', '123.456789', '12_345.6789%'),
 
             # negative zero: default behavior
             ('.1f', '-0', '-0.0'),

Misc/NEWS.d/next/Library/2025-04-06-14-34-29.gh-issue-130664.JF2r-U.rst

@@ -0,0 +1,2 @@
+Support ``'_'`` digit separator in formatting of the integral part of
+:class:`~decimal.Decimal`'s.  Patch by Sergey B Kirpichev.