Merge #15285: build: Prefer Python 3.4 even if newer versions are present on the system

0890339fb3 build: prefer python3.4 even if newer versions are present on the system (Sjors Provoost)

Pull request description:

  Python 3.4 is this mimimum supported version according to [doc/dependencies.md](https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md)

  Systems with [PyEnv](https://github.com/pyenv/pyenv) ensure (via [.python-version](https://github.com/bitcoin/bitcoin/blob/master/.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`.

  I missed this in #14884, so ideally this should be tagged 0.18

Tree-SHA512: b7487081a1ee7c2cb672a2e4bc1943ec8d23825fb941e567cb00fb123e6d59b1d8b7ddbf97d48aca770b9ddb9eacbfe73d8ac8cb1e1cdc34587ee1cee9929840

configure.ac

@@ -85,8 +85,8 @@ 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)