Add 'pip install pexpect' to travis config

In preparation for the new pexpect-based tests, modify
the travis config file to install pexpect.

.travis.yml

@@ -13,7 +13,10 @@ matrix:
             - gettext
             - libncurses5-dev
             - libpcre2-dev
-            - python
+            - python3
+            - python3-pip
+      before_install:
+        - sudo pip3 install pexpect
       env:
           # Some warnings upgraded to errors to match Open Build Service platforms
           - CXXFLAGS="-Werror=address -Werror=return-type"
@@ -26,7 +29,10 @@ matrix:
             - gettext
             - lib32ncurses5-dev
             - g++-multilib
-            - python
+            - python3
+            - python3-pip
+      before_install:
+        - sudo pip3 install pexpect
       env:
           - CXXFLAGS="-m32 -Werror=address -Werror=return-type" CFLAGS="-m32"
     - os: linux
@@ -35,6 +41,8 @@ matrix:
           - CXXFLAGS="-fno-omit-frame-pointer -fsanitize=undefined -fsanitize=address"
           - ASAN_OPTIONS=check_initialization_order=1:detect_stack_use_after_return=1:detect_leaks=1
           - UBSAN_OPTIONS=print_stacktrace=1:report_error_type=1:suppressions=$TRAVIS_BUILD_DIR/build_tools/ubsan.blacklist
+      before_install:
+        - sudo pip3 install pexpect
       addons:
         apt:
           packages:
@@ -43,10 +51,14 @@ matrix:
             - libncurses5-dev
             - libpcre2-dev
             - python
+            - python3
+            - python3-pip
     - os: linux
       compiler: clang
       env:
           - CXXFLAGS="-fsanitize=thread"
+      before_install:
+        - sudo pip3 install pexpect
       addons:
         apt:
           packages:
@@ -54,8 +66,11 @@ matrix:
             - gettext
             - libncurses5-dev
             - libpcre2-dev
-            - python
+            - python3
+            - python3-pip
     - os: osx
+      before_install:
+        - sudo pip3 install pexpect
   fast_finish: true
 
 script: