Use a wider and more modern set of impls for tests

Signed-off-by: Michał Górny <mgorny@gentoo.org>

Author: mgorny

Makefile.am

@@ -1,6 +1,6 @@
 AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--with-buffer-size=128 \
-	--with-python-impls='pypy python2.7 python3.3 python3.4 python3.5'
+	--with-python-impls='pypy pypy3 python2.7 python3.6 python3.7 python3.8 python3.9 python3.10'
 
 bin_PROGRAMS = python-exec2c
 scriptroot_SCRIPTS = python-exec2