reticulate 1.39.0

• Python background threads can now run in parallel with the R session (#1641).

• py_main_thread_func() is deprecated; every R function can now safely be
  called from background Python threads (#1648).

• Calls from Python threads into R now notify the main thread using R's native
  event loop, ensuring that these calls are handled even when the main thread
  is engaged in non-Python tasks (#1648).

• The knitr engine now avoids overwriting Altair's default chart dimensions with
  the values of ut.width.px and ut.height.px. Use altair.fig.height,
  altair.fig.width, or Altair's width and height parameters to adjust
  chart dimensions (contributed by @joelostblom, #1646).

• New as.character() method for python.builtin.str with support for handling
  embedded NULs in strings (#1653).

• New as.raw() method for python.builtin.bytes (#1649, #1652).

• as.character() method for python.builtin.bytes gains a nul argument,
  allowing for convenient handling of embedded NULs in the string (#1652).

• Reticulate now uses the RETICULATE_VIRTUALENV_ROOT environment variable when
  determining where to resolve virtual environments (#1657).

• conda_run2() is now exported (contributed by @dramanica, #1637).

• The Python session is now finalized when the R session exits (#1648).

• Internal updates for NumPy 2.1 (#1651).

• Fixed error when importing a module named config (#1628).

• Fixes for CRAN check failures on macOS-oldrel (#1645).

• Fixed an error where opening a Python subprocess in Positron on Windows
  resulted in "OSError: [WinError 6] The handle is invalid."
  (#1658, posit-dev/positron#4457).