Improve Python 3.7 compatibility.

async is now a full (non-contextual) keyword.

There are two more instances:
   - artiq/frontend/artiq_client.py
   - artiq/devices/thorlabs_tcube/driver.py

It is not immediately clear how to fix those, so they are left for
later work.

artiq/compiler/embedding.py

@@ -1019,7 +1019,7 @@ class Stitcher:
 
         function_type = types.TRPC(ret_type,
                                    service=self.embedding_map.store_object(host_function),
-                                   async=is_async)
+                                   is_async=is_async)
         self.functions[function] = function_type
         return function_type
 

artiq/compiler/transforms/llvm_ir_generator.py

@@ -1332,7 +1332,7 @@ class LLVMIRGenerator:
             llargptr = self.llbuilder.gep(llargs, [ll.Constant(lli32, index)])
             self.llbuilder.store(llargslot, llargptr)
 
-        if fun_type.async:
+        if fun_type.is_async:
             self.llbuilder.call(self.llbuiltin("rpc_send_async"),
                                 [llservice, lltagptr, llargs])
         else:
@@ -1342,7 +1342,7 @@ class LLVMIRGenerator:
         # Don't waste stack space on saved arguments.
         self.llbuilder.call(self.llbuiltin("llvm.stackrestore"), [llstackptr])
 
-        if fun_type.async:
+        if fun_type.is_async:
             # If this RPC is called using an `invoke` ARTIQ IR instruction, there will be
             # no other instructions in this basic block. Since this RPC is async, it cannot
             # possibly raise an exception, so add an explicit jump to the normal successor.

artiq/compiler/types.py

@@ -311,14 +311,14 @@ class TRPC(Type):
     :ivar ret: (:class:`Type`)
         return type
     :ivar service: (int) RPC service number
-    :ivar async: (bool) whether the RPC blocks until return
+    :ivar is_async: (bool) whether the RPC blocks until return
     """
 
     attributes = OrderedDict()
 
-    def __init__(self, ret, service, async=False):
+    def __init__(self, ret, service, is_async=False):
         assert isinstance(ret, Type)
-        self.ret, self.service, self.async = ret, service, async
+        self.ret, self.service, self.is_async = ret, service, is_async
 
     def find(self):
         return self
@@ -326,7 +326,7 @@ class TRPC(Type):
     def unify(self, other):
         if isinstance(other, TRPC) and \
                 self.service == other.service and \
-                self.async == other.async:
+                self.is_async == other.is_async:
             self.ret.unify(other.ret)
         elif isinstance(other, TVar):
             other.unify(self)
@@ -343,7 +343,7 @@ class TRPC(Type):
     def __eq__(self, other):
         return isinstance(other, TRPC) and \
                 self.service == other.service and \
-                self.async == other.async
+                self.is_async == other.is_async
 
     def __ne__(self, other):
         return not (self == other)
@@ -742,7 +742,7 @@ class TypePrinter(object):
                 return signature
         elif isinstance(typ, TRPC):
             return "[rpc{} #{}](...)->{}".format(typ.service,
-                                                 " async" if typ.async else "",
+                                                 " async" if typ.is_async else "",
                                                  self.name(typ.ret, depth + 1))
         elif isinstance(typ, TBuiltinFunction):
             return "<function {}>".format(typ.name)

artiq/coredevice/comm_kernel.py

@@ -403,7 +403,7 @@ class CommKernel:
             return msg
 
     def _serve_rpc(self, embedding_map):
-        async        = self._read_bool()
+        is_async     = self._read_bool()
         service_id   = self._read_int32()
         args, kwargs = self._receive_rpc_args(embedding_map)
         return_tags  = self._read_bytes()
@@ -413,9 +413,9 @@ class CommKernel:
         else:
             service  = embedding_map.retrieve_object(service_id)
         logger.debug("rpc service: [%d]%r%s %r %r -> %s", service_id, service,
-                     (" (async)" if async else ""), args, kwargs, return_tags)
+                     (" (async)" if is_async else ""), args, kwargs, return_tags)
 
-        if async:
+        if is_async:
             service(*args, **kwargs)
             return