support class "python.builtin.object" with "r_to_py" constructor

[moodymudskipper]

x <- reticulate::r_to_py(1:3)

# we want this:
construct(x)
#> reticulate::r_to_py(1:3)

x # x is actually an environment with a printing method
#> [1, 2, 3]    
construct(py_to_r(x)) # but we can construct the equivalent R object, so we have all we need 
#> 1:3  

General steps :

• Define a template using .cstr_new_class("python.builtin.object", constructor = "reticulate::r_to_py", commented = TRUE) that we save in the R folder as s3-python.builtin.object.R
• If we document, reload and retry construct(x) should already return something different.
• Adapt .cstr_construct.python.builtin.object.r_to_py() so construct(x) returns r_to_py(1:3), look at other examples in the package, here we can use idiomatic_class = class(x) to avoid class repair
• document and snapshot tests