travis CI: update meson image to one with a python that isn't EOL

Currently the build errors out with:

```
ERROR: This script does not work on Python 3.6 The minimum supported Python version is 3.7. Please use https://bootstrap.pypa.io/pip/3.6/get-pip.py instead.
```

While in theory this advice could be followed to get a better pip on
xenial, Meson has now deprecated python 3.6 support too, and the next
(unreleased) version requires python 3.7

There are a couple solutions to this:
- hold the version of pip, allow pip to only install 3.6-compatible
  versions of meson (effectively freezing meson going forward)
- install python 3.7 on xenial
- update to a 2-year-old image instead of a 4-year-old one

Option 3 is the simplest.

.travis.yml

@@ -92,8 +92,8 @@ matrix:
         - make -C tests versionsTest
 
     # meson dedicated test
-    - name: Xenial (Meson + clang)    # ~15mn
-      dist: bionic
+    - name: Focal (Meson + clang)    # ~15mn
+      dist: focal
       language: cpp
       compiler: clang
       install: