Allow running configure with python 3.4.

Details:
- Relax version blacklisting of python3 to allow 3.4 or later instead
  of 3.5 or later. Thanks to Dave Love for pointing out that 3.4 was
  sufficient for the purpose of BLIS's build system. (It should be
  noted that we're not sure which, if any, python3 versions prior to
  3.4 are insufficient, and that the only thing stopping us from
  determining this is the fact that these earlier versions of python3
  are not readily available for us to test with.)
- Updated docs/BuildSystem.md to be explicit about current python2 vs
  python3 version requirements.

configure

@@ -1214,7 +1214,13 @@ check_python()
 	#
 	# python1: no versions supported
 	# python2: 2.7+
-	# python3: 3.5+
+	# python3: 3.4+
+	#
+	# NOTE: It's actually unclear whether python 3.0 through 3.3.x would work.
+	# Python 3.5 is the oldest python3 that I have available to test with, and
+	# I only know that 3.4 will work thanks to feedback from Dave Love. So it's
+	# quite possible that some of those "unsupported" python3 versions are
+	# sufficient. -FGVZ
 	#
 
 	# Python 1.x is unsupported.
@@ -1229,9 +1235,9 @@ check_python()
 		fi
 	fi
 
-	# Python 3.4.x or older is unsupported.
+	# Python 3.3.x or older is unsupported.
 	if [ ${python_major} -eq 3 ]; then
-		if [ ${python_minor} -lt 5 ]; then
+		if [ ${python_minor} -lt 4 ]; then
 			echoerr_unsupportedpython
 		fi
 	fi

docs/BuildSystem.md

@@ -20,7 +20,7 @@ This document describes how to configure, compile, and install a BLIS library on
 
 The BLIS build system was designed for use with GNU/Linux (or some other sane UNIX). Other requirements are:
 
-  * Python (2.7 or later)
+  * Python (2.7 or later for python2; 3.4 or later for python3)
   * GNU `bash` (3.2 or later)
   * GNU `make` (3.81 or later)
   * a working C99 compiler