py/mkenv.mk: Change default PYTHON variable from "python" to "python3".

This change makes it so that python3 is required by default to build
MicroPython. Python 2 can be used by specifying make PYTHON=python2.

This comes about due to a recent-ish change to PEP 394 that makes the
python command more optional than before (even with Python 2 installed);
see python/peps@cd59ec0#diff-1d22f7bd72cbc900670f058b1107d426

Since the command python is no longer required to be provided by a
distribution we need to use either python2 or python3 as commands.  And
python3 seems the obvious choice.

README.md

@@ -60,7 +60,9 @@ Additional components:
 The subdirectories above may include READMEs with additional info.
 
 "make" is used to build the components, or "gmake" on BSD-based systems.
-You will also need bash, gcc, and Python (at least 2.7 or 3.3).
+You will also need bash, gcc, and Python 3.3+ available as the command `python3`
+(if your system only has Python 2.7 then invoke make with the additional option
+`PYTHON=python2`).
 
 The Unix version
 ----------------

py/mkenv.mk

@@ -42,7 +42,7 @@ ECHO = @echo
 CP = cp
 MKDIR = mkdir
 SED = sed
-PYTHON = python
+PYTHON = python3
 
 AS = $(CROSS_COMPILE)as
 CC = $(CROSS_COMPILE)gcc