Use string substitution in normalized_channel_url

[nkaretnikov]

Fixes #602.

Description

This pull request: appends the trailing slash to normalized_channel_url via string interpolation. The previous method of using the / operator was ignoring the trailing slash on Windows, which caused a channel_replacements lookup failure.

On Linux:

In [2]: yarl.URL("foo") / "./"
Out[2]: URL('foo/')

On Windows the slash is not appended, so the lookup in channel_replacements failed:

In [4]: yarl.URL("foo") / "./"
Out[4]: URL('foo')

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

Additional information

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[trallard]

Thank you @nkaretnikov - all seems to be in order and tests are passing so will approve and merge