build: require python-yaml >= 3.11.0

Add a configure check for PyYAML.  Version 3.11 was released
in 2014 and not much has changed since then, so it seems like
a good minimum version to start with.

From https://pyyaml.org/wiki/PyYAML:

3.13 (2018-07-05)
* Rebuild wheels using latest Cython for Python 3.7 support.

3.12 (2016-08-28)
* Wheel packages for Windows binaries.
* Adding an implicit resolver to a derived loader should not affect
  the base loader (fixes issue flux-framework#57).
* Uniform representation for OrderedDict across different versions
  of Python (fixes issue flux-framework#61).
* Fixed comparison to None warning (closes issue flux-framework#64).

3.11 (2014-03-26)
* Source and binary distributions are rebuilt against the latest
  versions of Cython and LibYAML.

configure.ac

@@ -232,6 +232,11 @@ AM_CHECK_PYMOD(six,
                ,
                [AC_MSG_ERROR([could not find python module six, version 1.9.0+ required])]
                )
+AM_CHECK_PYMOD(yaml,
+               [StrictVersion(yaml.__version__) >= StrictVersion ('3.11.0')],
+               ,
+               [AC_MSG_ERROR([could not find python module yaml, version 3.11+ required])]
+               )
 
 # Remove -L<path> from PYTHON_LDFLAGS if it is in a standard path
 # (e.g. /usr/lib64).  Placing a standard path earlier in the linker