feat(python): Support writing to file objects from write_excel

[alexander-beedie]

Closes #18849.

Allows write_excel to export to file objects as well as file paths (and BytesIO).

Example

from pathlib import Path
import polars as pl

df1 = pl.DataFrame({"colx": ["aaa", "bbb", "ccc"]})

# write to an open (binary-mode) file object
with Path("dataframe.xlsx").open(mode="wb") as p:
    df1.write_excel(p)

df2 = pl.read_excel(Path("dataframe.xlsx"))
df1.equals(df2)
# True

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.04%. Comparing base (247f0b1) to head (e261006).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/io/spreadsheet/_write_utils.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20638   +/-   ##
=======================================
  Coverage   79.03%   79.04%           
=======================================
  Files        1557     1557           
  Lines      220528   220545   +17     
  Branches     2510     2516    +6     
=======================================
+ Hits       174303   174324   +21     
+ Misses      45651    45646    -5     
- Partials      574      575    +1     

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