dev-libs/boost: Version bump to 1.70.0

dev-libs/boost: Version bump to 1.70.0

* bzip2/lzma/zlib/zstd support can now be
  disabled via USE flags explicitly.
* Upstream has fixed Boost.Python to finally
  support building against multiple Python 3
  implementations concurrently:

    https://github.com/boostorg/python/commit/d4d41d94aecc

  Going forward, Gentoo will stop modifying
  the upstream build system for its multiple
  implementations. This will lead to some
  short-term pain, as the library pattern changes
  from

    libboost_python-3.6.so

  to

    libboost_python36.so

  which is the canonical name used by upstream.
  Changing this name should be avoided, as the
  filename is also encoded as a macro in various
  boost headers.

Bug: https://bugs.gentoo.org/631590
Closes: https://bugs.gentoo.org/653878
Closes: https://github.com/gentoo/gentoo/pull/11659
Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: David Seifert <soap@gentoo.org>