Make multipart.boundary a str property

[asvetlov]

As discussed in #2545

@kxepal please review

[codecov-io]

Codecov Report

Merging #2589 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2589   +/-   ##
=======================================
  Coverage   97.87%   97.87%           
=======================================
  Files          38       38           
  Lines        7305     7305           
  Branches     1263     1263           
=======================================
  Hits         7150     7150           
  Misses         51       51           
  Partials      104      104

Impacted Files	Coverage Δ
aiohttp/multipart.py	95.47% <100%> (ø)	⬆️

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[kxepal]

LFGM. Ship it!

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new