Fix issue where string dtype singleton elsize was accidentally changed

[peytondmurray]

This fixes an issue where the numpy string/bytes dtype singletons were being modified by npyodbc. This happens in 2 places:

• This can occur when coercing SQL types to string/bytes numpy dtypes using the target_dtypes kwarg
• When reading binary column types (this is the case that prompted the original bug report)

A test was also added to catch this.