HTTP/[23]: continue upload when state.drain is set

- as reported in #10433, HTTP/2 uploads may stall when a response is
  received before the upload is done. This happens when the
  data->state.drain is set for such a transfer, as the special handling
  in transfer.c from then on only cared about downloads.
- add continuation of uploads, if applicable, in this case.
- add pytest case test_07_12_upload_seq_large to reproduce this scenario
  (although, current nghttp2 implementation is using drain less often)

Reported-by: Lucas Pardue

Fixes #10433
Closes #10443

lib/transfer.c

@@ -1080,6 +1080,8 @@ CURLcode Curl_readwrite(struct connectdata *conn,
   if(data->state.drain) {
     select_res |= CURL_CSELECT_IN;
     DEBUGF(infof(data, "Curl_readwrite: forcibly told to drain data"));
+    if((k->keepon & KEEP_SENDBITS) == KEEP_SEND)
+      select_res |= CURL_CSELECT_OUT;
   }
 #endif