tensorflow 2.6.0

Updated default Tensorflow version to 2.6.
Changed default in tf_function() to autograph=TRUE.
Added S3 generic as_tensor().
Refactored install_tensorflow().
- Potentially breaking change: numeric versions supplied without a patchlevel now automatically pull the latest patch release.
  (e.g. install_tensorflow(version="2.4") will install "2.4.2". Previously it would install "2.4.0")
Removed "Config/reticulate" declaration from DESCRIPTION.
- Setting RETICULATE_AUTOCONFIGURE=FALSE environment variable when using non-default tensorflow installations (e.g., 'tensorflow-cpu') no longer required.
- Users will have to call install_tensorflow() for automatic installation.
tfautograph added to Imports
jsonlite removed from Imports, tfestimators removed from Suggests
Refactored automated tests to closer match the default installation procedure
and compute environment of most user.
Expanded CI test coverage to include R devel, oldrel and 3.6.
Fixed an issue where extra packages with version constraints like
install_tensorflow(extra_packages = "Pillow<8.3") were not quoted properly.
Fixed an issue where valid tensor-like objects supplied to
log(x, base), cospi(), tanpi(), and sinpi() would raise an error.

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