Reading the value of py forces python initialization

[dfalbel]

This PR fixes #1334

It fixes by forcing py to initialize Python. We could save objects to a temporary environment until Python is initialized, but this could delay some r_to_py() cast errors which would be hard to debug.

py has been added in #107 and there doesn't seem to be any discussions about why it would be undesirable for it to start python.

[t-kalinowski]

I think we need to revert this. While tracking down #1450, I inserted a debugging call to print(rlang::trace_back()) in create_default_virtualenv() and saw this when installing the package.

==> R CMD INSTALL --no-multiarch --with-keep.source reticulate

* installing to library ‘/Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library’
* installing *source* package ‘reticulate’ ...
** using staged installation
** libs
using C++ compiler: ‘Apple clang version 14.0.3 (clang-1403.0.22.14.1)’
using SDK: ‘MacOSX13.3.sdk’
make: Nothing to be done for `all'.
installing to /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library/00LOCK-reticulate/00new/reticulate/libs
** R
** inst
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
*** copying figures
** building package indices
** installing vignettes
** testing if installed package can be loaded from temporary location
** checking absolute paths in shared objects and dynamic libraries
** testing if installed package can be loaded from final location
     ▆
  1. ├─base::suppressWarnings(...)
  2. │ └─base::withCallingHandlers(...)
  3. ├─base::serialize(...)
  4. ├─base::as.list(base::getNamespace("reticulate"), all.names = TRUE)
  5. ├─base::as.list.environment(base::getNamespace("reticulate"), all.names = TRUE)
  6. └─reticulate (local) `<fn>`()
  7.   └─reticulate::import_main(convert = TRUE) at reticulate/R/zzz.R:25:4
  8.     └─reticulate::import("__main__", convert = convert, delay_load = delay_load) at reticulate/R/import.R:161:2
  9.       └─reticulate:::ensure_python_initialized(required_module = module) at reticulate/R/import.R:97:4
 10.         └─reticulate:::initialize_python() at reticulate/R/package.R:60:2
 11.           └─(function() {... at reticulate/R/package.R:121:2
 12.             └─reticulate::py_discover_config(required_module, use_environment) at reticulate/R/package.R:124:4
 13.               └─reticulate:::create_default_virtualenv(package = "reticulate") at reticulate/R/config.R:322:2
** testing if installed package keeps a record of temporary installation path
* DONE (reticulate)

It seems that testing the package namespace as part of installation evaluates the active binding py, even though a regular library(reticulate) call does not.