Fix invalid conversion from 'const char8_t*' to 'const char*'

Benjamin Beasleyrequested to merge
musicinmybrain/graph-tool:gcc16 into master

This showed up in Fedora with the update to (a pre-release of) GCC 16 in Fedora Rawhide.

In file included from src/graph/graph_io.cc:91:
src/graph/graph_io_binary.hh:30:32: error: invalid conversion from 'const char8_t*' to 'const char*' [-fpermissive]
   30 | constexpr const char* _magic = u8"⛾ gt";
      |                                ^~~~~~~~
      |                                |
      |                                const char8_t*

Instead, I just wrote out the UTF-8 encoded bytes, "\xe2\x9b\xbe gt".

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