INSTALL: document Python 3 as a build tool

Needed for scripts/process_function_template.py run by
src/core/CMakeLists.txt.

Enforced since commit:
f16fe73 ("Python is a hard build time dependency", 2015-09-20)

INSTALL

@@ -1,10 +1,10 @@
 QGIS
 Building QGIS from source - step by step
-Monday April 15, 2019
+Tuesday May 07, 2019
 
 
-Last Updated: Monday April 15, 2019
-Last Change : Monday April 15, 2019
+Last Updated: Tuesday May 07, 2019
+Last Change : Tuesday May 07, 2019
 
 
   1. Introduction
@@ -94,6 +94,7 @@ Required build tools:
 - CMake >= 3.0.0
 - Flex >= 2.5.6
 - Bison >= 2.4
+- Python 3
 
 Required build dependencies:
 

doc/INSTALL.html

@@ -5,7 +5,7 @@
 <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
 <TITLE>QGIS</TITLE>
 
-<!-- Included /home/fischer/src/qgis/doc/style.css -->
+<!-- Included doc/style.css -->
 <STYLE TYPE="text/css">
 body{  background: white;
   color: black;
@@ -77,13 +77,13 @@
 <DIV CLASS="header" ID="header">
 <H1>QGIS</H1>
 <H2>Building QGIS from source - step by step</H2>
-<H3>Monday April 15, 2019</H3>
+<H3>Tuesday May 07, 2019</H3>
 </DIV>
 
 <DIV CLASS="body" ID="body">
 <P>
-Last Updated: Monday April 15, 2019
-Last Change : Monday April 15, 2019
+Last Updated: Tuesday May 07, 2019
+Last Change : Tuesday May 07, 2019
 </P>
 <DIV CLASS="toc">
 
@@ -196,6 +196,7 @@ <H1>2. Overview</H1>
 <LI>CMake &gt;= 3.0.0
 <LI>Flex &gt;= 2.5.6
 <LI>Bison &gt;= 2.4
+<LI>Python 3
 </UL>
 
 <P>
@@ -3125,5 +3126,5 @@ <H1>9. Authors and Acknowledgments</H1>
 
 </DIV>
 <!-- html code generated by txt2tags 2.6 (http://txt2tags.org) -->
-<!-- cmdline: txt2tags -\-encoding=utf-8 -o/home/fischer/src/qgis/debian/build/doc/INSTALL.html -t html /home/fischer/src/qgis/doc/INSTALL.t2t -->
+<!-- cmdline: txt2tags -\-encoding=utf-8 -o doc/INSTALL.html -t html doc/INSTALL.t2t -->
 </BODY></HTML>

doc/overview.t2t

@@ -11,6 +11,7 @@ Required build tools:
 - CMake >= 3.0.0
 - Flex >= 2.5.6
 - Bison >= 2.4
+- Python 3
 -
 
 Required build dependencies: