avoided potential type's mismatch

[igeni]

avoided potential type's mismatch during concatenation and remove redundant type's conversion

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Description of your pull request and other information

Avoided potential type's mismatch during concatenation and remove redundant type's conversion

[dirkf]

What problem is being solved here?

You can't use f'{x}{y}' since the code should be able to run on versions of Python that treat that as a syntax error. If such a change should be needed, you would have to use '{0}{1}'.format(x, y). Apparently that would be OK here but elsewhere the code enables unicode_literals, with which the '...'.format(...) code would incorrectly generate a unicode value instead of a str in Python2.