build: add python3.10 alias to AC_PATH_PROGS call in configure

Python 3.10 is now relased, and has been available as a beta/rc in
distros for a little while already.

configure.ac

@@ -107,7 +107,7 @@ AC_PATH_TOOL(GCOV, gcov)
 AC_PATH_TOOL(LLVM_COV, llvm-cov)
 AC_PATH_PROG(LCOV, lcov)
 dnl Python 3.6 is specified in .python-version and should be used if available, see doc/dependencies.md
-AC_PATH_PROGS([PYTHON], [python3.6 python3.7 python3.8 python3.9 python3 python])
+AC_PATH_PROGS([PYTHON], [python3.6 python3.7 python3.8 python3.9, python3.10, python3 python])
 AC_PATH_PROG(GENHTML, genhtml)
 AC_PATH_PROG([GIT], [git])
 AC_PATH_PROG(CCACHE,ccache)