Lxml etree serialzation io_encoder error workaround

[sgniedoj]

Description

Adds "encoding" parameter to MailMerge.write

Motivation and Context

Calling MailMerge.write(file) occasionally causes lxml.etree.SerialisationError: IO_ENCODER exception.
Adding encoding='UTF-8' to xml = etree.tostring(self.parts[zi].getroot(), encoding='UTF-8') eliminates the problem.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.