sage --fixdoctests does not work with line continuation

[user202729]

Steps To Reproduce

a.sage:

r"""
sage: 123 + \
....: 456
578
"""

run

$ sage --fixdoctests --overwrite a.sage

Expected Behavior

replace

Actual Behavior

.../src/bin/sage-fixdoctests:351: UserWarning: Did not manage to replace
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
    578
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
with
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
    579
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  warnings.warn(txt % ('>' * 40, '\n'.join(expected), '>' * 40,
sage-fixdoctests: No fixes made in 'a.sage'

Additional Information

No response

Environment

• OS: Linux
• Sage Version: approximately latest

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide