build: refactor configure.py

- Explicitly specify the encoding when opening files.
- Use f-strings to format strings.
- Use `isinstance()` for type checks instead of `type()`.
- Use the `with` keyword for resource-allocating operations.
- Avoid using multiple statements in a single line.
- Remove unnecessary `else` clauses after `return`.
- Iterate with the `items()` method of dictionaries when both the key
and value are used.
- Remove unnecessary parentheses.
- Rename unused unpacked variables to `_`, `_1`, etc etc.
- Rename the `list` variable to avoid conflict with the global
`list()` function.
- Remove unused `path` parameter of the `icu_download()` function.
- Remove unused return value of the `configure_section_file()`
function.