Travis: Make Windows build more reliable by trying different key servers

contrib/build-wine/prepare-wine.sh

@@ -54,6 +54,27 @@ download_if_not_exist() {
     fi
 }
 
+# https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/templates/header.sh
+retry() {
+  local result=0
+  local count=1
+  while [ $count -le 3 ]; do
+    [ $result -ne 0 ] && {
+      echo -e "\nThe command \"$@\" failed. Retrying, $count of 3.\n" >&2
+    }
+    ! { "$@"; result=$?; }
+    [ $result -eq 0 ] && break
+    count=$(($count + 1))
+    sleep 1
+  done
+
+  [ $count -gt 3 ] && {
+    echo -e "\nThe command \"$@\" failed 3 times.\n" >&2
+  }
+
+  return $result
+}
+
 # Let's begin!
 here=$(dirname $(readlink -e $0))
 set -e
@@ -74,8 +95,8 @@ cd /tmp/electrum-build
 # keys from https://www.python.org/downloads/#pubkeys
 KEYLIST_PYTHON_DEV="531F072D39700991925FED0C0EDDC5F26A45C816 26DEA9D4613391EF3E25C9FF0A5B101836580288 CBC547978A3964D14B9AB36A6AF053F07D9DC8D2 C01E1CAD5EA2C4F0B8E3571504C367C218ADD4FF 12EF3DC38047DA382D18A5B999CDEA9DA4135B38 8417157EDBE73D9EAC1E539B126EB563A74B06BF DBBF2EEBF925FAADCF1F3FFFD9866941EA5BBD71 2BA0DB82515BBB9EFFAC71C5C9BE28DEE6DF025C 0D96DF4D4110E5C43FBFB17F2D347EA6AA65421D C9B104B3DD3AA72D7CCB1066FB9921286F5E1540 97FC712E4C024BBEA48A61ED3A5CA953F73C700D 7ED10B6531D7C8E1BC296021FC624643487034E5"
 KEYRING_PYTHON_DEV="keyring-electrum-build-python-dev.gpg"
-KEYSERVER_PYTHON_DEV="hkp://keys.gnupg.net"
-gpg --no-default-keyring --keyring $KEYRING_PYTHON_DEV --keyserver $KEYSERVER_PYTHON_DEV --recv-keys $KEYLIST_PYTHON_DEV
+KEYSERVER_PYTHON_DEV="hkp://pool.sks-keyservers.net"
+retry gpg --no-default-keyring --keyring $KEYRING_PYTHON_DEV --keyserver $KEYSERVER_PYTHON_DEV --recv-keys $KEYLIST_PYTHON_DEV
 for msifile in core dev exe lib pip tools; do
     echo "Installing $msifile..."
     wget -nc "https://www.python.org/ftp/python/$PYTHON_VERSION/win32/${msifile}.msi"