[Feature]: sidecar Support Text Output to io.StringIO()

[MAbdElRaouf]

Describe the proposed feature

Using sidecar="page_text.txt" works great and the generated file on disk has the expected text. However, passing an io.StringIO() to sidecar doesn't seem to work and no text is being saved to the buffer:

import ocrmypdf
import io

output_pdf_file_obj = io.BytesIO()

page_text_buffer = io.StringIO()

ocrmypdf.ocr(
    "source_pdf.pdf",
    output_pdf_file_obj,
    sidecar=page_text_buffer,
    pages=1,
    tesseract_pagesegmode=1,
)

>>> page_text_buffer.seek(0)
>>> print(page_text_buffer.read())
''

>>> page_text_buffer.seek(0)
>>> print(page_text_buffer.getvalue())
''