Add Python 3.8 support: use CodeType.replace() if available

Backport upstream change:
806713333a

Remove also 0001-always-obey-GEVENT_NO_CFFI_BUILD.patch, the change
has bene merged into gevent 1.3:
14b28c678f

0001-code-replace.patch

@@ -0,0 +1,26 @@
+commit 1b84da94c2aa2d171881482c22298ad189b79764
+Author: Victor Stinner <vstinner@redhat.com>
+Date:   Tue Jun 11 16:38:52 2019 +0200
+
+    Use CodeType.replace() if available
+    
+    Port _tblib.Traceback to Python 3.8: use code.replace() method if
+    available.
+
+diff --git a/src/gevent/_tblib.py b/src/gevent/_tblib.py
+index 13364012..b9553a9e 100644
+--- a/src/gevent/_tblib.py
++++ b/src/gevent/_tblib.py
+@@ -198,7 +198,11 @@ class Traceback(object):
+         while current:
+             f_code = current.tb_frame.f_code
+             code = compile('\n' * (current.tb_lineno - 1) + 'raise __traceback_maker', current.tb_frame.f_code.co_filename, 'exec')
+-            if PY3:
++            if hasattr(code, "replace"):
++                # Python 3.8 and newer
++                code = code.replace(co_argcount=0,
++                                    co_freevars=(), co_cellvars=())
++            elif PY3:
+                 code = CodeType(
+                     0, code.co_kwonlyargcount,
+                     code.co_nlocals, code.co_stacksize, code.co_flags,