Signed/unsigned conversion in Boost.Python

Using Boost v1.90

In `boost/include/boost/python/suite/indexing/detail/indexing_suite_detail.hpp` there are locals that have a signed/unsigned mismatch.

On L176, a local variable `index` declared as `long` is assigned to a return that can be unsigned (`size_t`).

Similarly, on L611, another local variable `from` is declared as `long` then `operator+=` is used for a variable that has an unsigned type.

This results in an unnecessarily wordy warning in the compiler (in my case msvc 14.2)

Inspect boost/include/boost/python/suite/indexing/detail/indexing_suite_detail.hpp at lines 176 and 611, checking the types returned or modified by the referenced operations. Update the signed/unsigned handling without changing behavior, then build or run the relevant Boost.Python checks and confirm the compiler warnings are resolved.

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