Use Python 3 for httpbin installation under Ubuntu Trusty

Trying to use Python 2 under Trusty is hopeless, as pythonhosted.org has
already started dropping support for non-SNI clients and will drop it
completely soon, see https://status.python.org/incidents/hzmjhqsdjqgb
and https://github.com/pypa/pypi-support/issues/978, and SNI is not
available in the system Python 2 version (2.7.6, which is less than
2.7.9 in which it was added).

Note that we still can't use Python 3 everywhere as long as we have
macOS 11 builds as it's not available there.

build/tools/httpbin.sh

@@ -17,44 +17,45 @@ httpbin_launch() {
 
     case "$dist_codename" in
         trusty)
-            # Current decorator (>= 5) is incompatible with Python 2 which we
-            # still use in some builds, so explicitly use a version which
-            # is known to work. Just specifying "decorator==4.4.2" doesn't work
-            # neither for some reason ("no matching distribution found"), so
-            # use an explicit URL.
-            pip_decorator_arg='https://files.pythonhosted.org/packages/ed/1b/72a1821152d07cf1d8b6fce298aeb06a7eb90f4d6d41acec9861e7cc6df0/decorator-4.4.2-py2.py3-none-any.whl'
+            # Python 2.7.6 is too old to support SNI and can't be used.
+            PY3=3
 
-            # We need to disable SSL certificate checking under Trusty because
-            # Python version there is too old to support SNI.
-            pip_options='--trusted-host files.pythonhosted.org'
+            # Current decorator (>= 5) is incompatible with Python 3.4 used
+            # here, so explicitly use a version which is known to work.
+            pip_decorator_arg='decorator==4.4.2'
             ;;
 
         macOS)
+            # We use Python 2 under macOS 10.11 which doesn't have Python 3,
+            # and decorator >= 5 is incompatible with it too.
             pip_decorator_arg='decorator==4.4.2'
             ;;
 
         *)
             # Elsewhere just use Python 3.
             PY3=3
-
-            # 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
-            fi
-
-            # Running pip install fails with weird errors out of the box when
-            # using old pip version because it attempts to use python rather
-            # than python3, so upgrade it to fix this.
-            #
-            # However don't upgrade to a version which is too new because then
-            # it may not support Python version that we actually have.
-            python3 -m pip install --user --upgrade pip==20.0.1
             ;;
     esac
 
+    if [ "$PY3" = 3 ]; then
+        # Ensure that we have at least some version of pip.
+        if ! python3 -m pip; then
+            sudo apt-get -q -o=Dpkg::Use-Pty=0 install python3-pip
+        fi
+
+        # Running pip install fails with weird errors out of the box when
+        # using old pip version because it attempts to use python rather
+        # than python3, so upgrade it to fix this.
+        #
+        # However don't upgrade to a version which is too new because then
+        # it may not support Python version that we actually have (this one
+        # still works with 3.4, 20.0.1 is the last one to support 3.5).
+        python3 -m pip install --user --upgrade pip==19.1.1
+    fi
+
     echo "Installing using `python$PY3 -m pip --version`"
 
-    python$PY3 -m pip install $pip_decorator_arg httpbin --user $pip_options
+    python$PY3 -m pip install $pip_decorator_arg httpbin --user
     python$PY3 -m httpbin.core 2>&1 >httpbin.log &
     WX_TEST_WEBREQUEST_URL="http://localhost:5000"