utf-8 encoding for schema file

[GrixaYrev]

Explicitly specify encoding (utf-8) when open schema file.
It is need if non-ASCII strings in json schema file.

[michamos]

@GrixaYrev thanks for your PR! Which OS and version of Python are you using? What does locale.getencoding() return? If no encoding is specified, it should fall back to a default encoding according to the docs, which is UTF-8 in many cases.

[GrixaYrev]

@michamos, I use Windows, and locale.getencoding() return "cp1251" according to the docs:

On Windows, return the ANSI code page.

I think we can use utf-8 for converter unconditionally, because content is not printed to terminal.