add "# cython: language_level=3" directive to .pyx files, fixes #4214

in master branch, it was possible to do this via setup.py's
cythonize() calls, but we do not have them in 1.1.

thus, i modified the source files, setting the language level to 3,
which is supported since long in Cython (3str is only supported since
recently so might fail with older Cython versions).

Before this changeset, current Cython 0.29.x emitted a FutureWarning,
but still compiled. The next major release of Cython is said to emit
an error instead, so this changeset hopefully avoids the build breaking
after a Cython upgrade.

src/borg/algorithms/checksums.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 from ..helpers import bin_to_hex
 
 from libc.stdint cimport uint32_t

src/borg/chunker.pyx

@@ -1,4 +1,4 @@
-# -*- coding: utf-8 -*-
+# cython: language_level=3
 
 API_VERSION = '1.1_01'
 

src/borg/compress.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 """
 borg.compress
 =============

src/borg/crypto/low_level.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 """A thin OpenSSL wrapper"""
 
 import hashlib

src/borg/hashindex.pyx

@@ -1,4 +1,5 @@
-# -*- coding: utf-8 -*-
+# cython: language_level=3
+
 from collections import namedtuple
 import locale
 import os

src/borg/item.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 import stat
 from collections import namedtuple
 

src/borg/platform/darwin.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 import os
 
 from ..helpers import user2uid, group2gid

src/borg/platform/freebsd.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 import os
 
 from ..helpers import posix_acl_use_stored_uid_gid

src/borg/platform/linux.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 import os
 import re
 import stat

src/borg/platform/posix.pyx

@@ -1,3 +1,5 @@
+# cython: language_level=3
+
 import errno
 import os