Switch build-time tests and autopkgtests to Python 3

debian/control

@@ -36,8 +36,8 @@ Build-Depends:
  libsystemd-dev,
  libtool,
  procps,
- python <!nocheck>,
- python-yaml <!nocheck>,
+ python3 <!nocheck>,
+ python3-yaml <!nocheck>,
  xsltproc <!nodoc>,
  zlib1g-dev,
 Build-Depends-Indep:
@@ -155,8 +155,8 @@ Depends:
  gir1.2-ostree-1.0,
  gjs [!sparc64],
  ostree,
- python,
- python-yaml,
+ python3,
+ python3-yaml,
  ${misc:Depends},
  ${shlibs:Depends},
 Description: content-addressed filesystem for operating system binaries - tests