build: prefer python3.4 even if newer versions are present on the system

Python 3.4 is the mimimum supported version according to doc/dependencies.md

Systems with PyEnv ensure (via .python-version) that Python 3.4 is used
for the functional tests. However make check calls bitcoin-util-test.py
using the Python command found by configure.ac, which looks system wide.

On systems with multiple versions of Python this would cause make check
to fail, as it tries to call a version of Python that PyEnv blocks.

This is solved by preferring python3.4 in configure.ac

configure.ac

@@ -84,8 +84,8 @@ AC_PATH_TOOL(RANLIB, ranlib)
 AC_PATH_TOOL(STRIP, strip)
 AC_PATH_TOOL(GCOV, gcov)
 AC_PATH_PROG(LCOV, lcov)
-dnl Python 3.x is supported from 3.4 on (see https://github.com/bitcoin/bitcoin/issues/7893)
-AC_PATH_PROGS([PYTHON], [python3.7 python3.6 python3.5 python3.4 python3 python])
+dnl Python 3.4 is specified in .python-version and should be used if available, see doc/dependencies.md
+AC_PATH_PROGS([PYTHON], [python3.4 python3.5 python3.6 python3.7 python3 python])
 AC_PATH_PROG(GENHTML, genhtml)
 AC_PATH_PROG([GIT], [git])
 AC_PATH_PROG(CCACHE,ccache)