Disable wxWebRequest tests on CI platforms without Python 3

This currently means macOS 10.11 only, which is only used on Travis CI
and Python 2 installation there is broken anyhow, so this doesn't make
anything worse than it already is.

build/tools/httpbin.sh

@@ -3,6 +3,15 @@
 # Do not run it directly.
 
 httpbin_launch() {
+    WX_TEST_WEBREQUEST_URL=0
+    export WX_TEST_WEBREQUEST_URL
+
+    # We need python3 for httpbin, python2 can't be used any longer.
+    if ! command -v python3 > /dev/null; then
+        echo 'Python 3 is not available, not using httpbin.'
+        return
+    fi
+
     echo 'Launching httpbin...'
 
     case "$(uname -s)" in
@@ -17,9 +26,6 @@ httpbin_launch() {
 
     case "$dist_codename" in
         trusty)
-            # Python 2.7.6 is too old to support SNI and can't be used.
-            PY3=3
-
             # Explicitly select the versions of dependencies that are still
             # compatible with Python 3.4 used here as the latest versions of
             # several of them are not.
@@ -33,14 +39,8 @@ httpbin_launch() {
             # and decorator >= 5 is incompatible with it too.
             pip_explicit_deps='decorator==4.4.2'
             ;;
-
-        *)
-            # Elsewhere just use Python 3.
-            PY3=3
-            ;;
     esac
 
-    if [ "$PY3" = 3 ]; then
     # Ensure that we have at least some version of pip.
     if ! python3 -m pip; then
         sudo apt-get -q -o=Dpkg::Use-Pty=0 install python3-pip
@@ -54,20 +54,19 @@ httpbin_launch() {
     # it may not support Python version that we actually have (this one
     # still works with 3.4, 20.0.1 is the last one to support 3.5).
     python3 -m pip install --user --upgrade pip==19.1.1
-    fi
 
-    echo "Installing using `python$PY3 -m pip --version`"
+    echo "Installing using `python3 -m pip --version`"
 
-    python$PY3 -m pip install $pip_explicit_deps httpbin --user
+    python3 -m pip install $pip_explicit_deps httpbin --user
-    python$PY3 -m httpbin.core 2>&1 >httpbin.log &
+    python3 -m httpbin.core 2>&1 >httpbin.log &
     WX_TEST_WEBREQUEST_URL="http://localhost:5000"
-
-    export WX_TEST_WEBREQUEST_URL
 }
 
 httpbin_show_log() {
+    if [ "$WX_TEST_WEBREQUEST_URL" != "0" ]; then
         echo '*** Tests failed, contents of httpbin.log follows: ***'
         echo '-----------------------------------------------------------'
         cat httpbin.log
         echo '-----------------------------------------------------------'
+    fi
 }