doc: update status of Python 3 support

Refs: nodejs/TSC#781 (comment)
Based on the completion of the Python 3 strategic
initiative, update statements around Python 3 support.

Author: mhdawson

BUILDING.md

@@ -223,18 +223,17 @@ Consult previous versions of this document for older versions of Node.js:
 
 ### Note about Python 2 and Python 3
 
-The Node.js project uses Python as part of its build process and has
+The Node.js project uses Python as part of its build process and was
 historically only been Python 2 compatible.
 
 Python 2 will reach its _end-of-life_ at the end of 2019 at which point the
 interpreter will cease receiving updates. See https://python3statement.org/
 for more information.
 
-The Node.js project is in the process of transitioning its Python code to
-Python 3 compatibility. Installing both versions of Python while building
-and testing Node.js allows developers and end users to test, benchmark,
-and debug Node.js running on both versions to ensure a smooth and complete
-transition before the year-end deadline.
+The Node.js project now supports both Python 3 and Python 2 for building.
+If both are installed Python 3 will be used. If only Python 2 is available
+it will be used instead. When possible we recommend that you build and
+test with Python 3.
 
 ### Unix and macOS
 
@@ -244,7 +243,7 @@ transition before the year-end deadline.
 * GNU Make 3.81 or newer
 * Python (see note above)
   * Python 2.7
-  * Python 3.5, 3.6, 3.7, and 3.8 are experimental.
+  * Python 3.5, 3.6, 3.7, and 3.8.
 
 Installation via Linux package manager can be achieved with:
 
@@ -260,7 +259,7 @@ FreeBSD and OpenBSD users may also need to install `libexecinfo`.
 * Xcode Command Line Tools >= 10 for macOS
 * Python (see note above)
   * Python 2.7
-  * Python 3.5, 3.6, 3.7, and 3.8 are experimental.
+  * Python 3.5, 3.6, 3.7, and 3.8.
 
 macOS users can install the `Xcode Command Line Tools` by running
 `xcode-select --install`. Alternatively, if you already have the full Xcode