python3Packages.tensorflow_2: 2.3.2 -> 2.4.0

Also the following related changes:
* Removed Python 2 support because it's not supported by TF and related packages for a long time.
* Upgraded tensorboard and estimator packages to the required versions.
* Added extra plugins for tensorboard to support profiling.
* In the previous derivation versions, TF_SYSTEM_LIBS didn't have any effect because it was reset at repo fetching stage, so TF always used its own dependencies. Made TF_SYSTEM_LIBS actually work and fixed the errors caused by enabling it.
* Enabled tensorboard by default (but still keeping an option to disable it if needed).

pkgs/development/python-modules/tensorboard-plugin-wit/default.nix

@@ -0,0 +1,22 @@
+{ lib, fetchPypi, buildPythonPackage
+}:
+
+buildPythonPackage rec {
+  pname = "tensorboard_plugin_wit";
+  version = "1.7.0";
+  format = "wheel";
+
+  src = fetchPypi {
+    inherit pname version;
+    format = "wheel";
+    python = "py3";
+    sha256 = "0nv855qm2fav70lndsrv810pqgg41sbmd70fk86wk18ih825yxzf";
+  };
+
+  meta = with lib; {
+    description = "What-If Tool TensorBoard plugin.";
+    homepage = http://tensorflow.org;
+    license = licenses.asl20;
+    maintainers = with maintainers; [ ndl ];
+  };
+}