Enforce Python 3 as much as possible

Author: GuillaumeGomez

README.md

@@ -27,7 +27,7 @@ or reading the [rustc dev guide][rustcguidebuild].
 1. Make sure you have installed the dependencies:
 
    * `g++` 5.1 or later or `clang++` 3.5 or later
-   * `python` 2.7 (but not 3.x)
+   * `python` 3 or 2.7
    * GNU `make` 3.81 or later
    * `cmake` 3.4.3 or later
    * `curl`
@@ -113,7 +113,7 @@ build.
                make \
                diffutils \
                tar \
-               mingw-w64-x86_64-python2 \
+               mingw-w64-x86_64-python \
                mingw-w64-x86_64-cmake \
                mingw-w64-x86_64-gcc
    ```

config.toml.example

@@ -168,11 +168,9 @@
 
 # Python interpreter to use for various tasks throughout the build, notably
 # rustdoc tests, the lldb python interpreter, and some dist bits and pieces.
-# Note that Python 2 is currently required.
 #
-# Defaults to python2.7, then python2. If neither executable can be found, then
-# it defaults to the Python interpreter used to execute x.py.
-#python = "python2.7"
+# Defaults to the Python interpreter used to execute x.py.
+#python = "python"
 
 # Force Cargo to check that Cargo.lock describes the precise dependency
 # set that all the Cargo.toml files create, instead of updating it.