tensorflow 2.20.0

Updates for reticulate 1.41 and reticuate::py_require().
New function py_require_tensorflow() which can be called at the start of an R session
to enable reticulate to resolve tensorflow. Calling install_tensorflow()
in most circumstances is no longer necessary.
install_tensorflow() installs TensorFlow v2.20 by default.
Fixed an issue where GPUs would not be found when running on Windows
WSL Linux (reported in rstudio/keras3#1456, fixed in #599)
Fixes for NumPy 2.0 (#601)
Fixes for R-devel (4.5)

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