Update context.cpp to comply with pybind11 2.0+ docs

[andriish]

Update context.cpp to comply with pybind11 2.0+ docs, i.e.
PYBIND11_DECLARE_HOLDER_TYPE from version 2.0 requires at least 3 arguments otherwise the compillers will issue warnings.
See https://pybind11.readthedocs.io/en/stable/advanced/smart_ptrs.html

The first argument of PYBIND11_DECLARE_HOLDER_TYPE() should be a placeholder name that is used as a template parameter of the second argument. Thus, feel free to use any identifier, but use it consistently on both sides; also, don’t use the name of a type that already exists in your codebase.

The macro also accepts a third optional boolean parameter that is set to false by default. 

[andriish]

Also contains #296

[lyskov]

LGTM, - thank you @andriish !