[Build] Migrate to Python3 (#1000)

* Configure Travis build env for Py3

Using default available version of 3.6.3 in `xenial` dist

* Migrate to py3

* Fix documentation

.travis.yml

@@ -16,6 +16,9 @@ language: cpp
 sudo: false
 compiler:
   - clang
+install:
+  - pyenv install 3.6.3
+  - pyenv global 3.6.3
 before_script:
   - CHECKOUT_DIR=$PWD && cd .. 
   - chmod a+x $CHECKOUT_DIR/support/checkout-deps.sh 
@@ -23,5 +26,5 @@ before_script:
 script:
   - mkdir build && cd build
   - PATH="~/.local/bin:$PATH"
-  - CC=clang-3.7 CXX=clang-3.7 python ../configure.py --enable-optimize
+  - CC=clang-3.7 CXX=clang-3.7 python3 ../configure.py --enable-optimize
   - ambuild

support/checkout-deps.sh

@@ -92,19 +92,15 @@ repo="https://github.com/alliedmodders/hlsdk"
 origin=
 checkout
 
-python_cmd=`command -v python`
-if [ -z "$python_cmd" ]; then
 python_cmd=`command -v python3`
-
 if [ -z "$python_cmd" ]; then
-    echo "No suitable installation of Python detected"
+  echo "No suitable installation of Python detected. Min required is 3.6"
   exit 1
 fi
-fi
 
 `$python_cmd -c "import ambuild2"` 2>&1 1>/dev/null
 if [ $? -eq 1 ]; then
-  echo "AMBuild is required to build SourceMod"
+  echo "AMBuild is required to build AMXModX"
 
   `$python_cmd -m pip --version` 2>&1 1>/dev/null
   if [ $? -eq 1 ]; then