Add Cirrus CI (#7431)

* Cirrus: Add Tox task

* Cirrus: Add Locale task

* Cirrus: Add Flake8 Task

* Cirrus: Add Regtest task

* Regtest: Flush stdout

Allows viewing output sooner.

* Regtest: Read process.stdout in text mode

Improves ability to quickly see output.

* Cirrus: Add Windows task

* Cirrus: Add Android task

* Cirrus: Add macOS task

* Cirrus: Add AppImage task

* Cirrus: Add tarball task

* Cirrus: Add Submodules task

* Android: remove superfluous cp/rm

* Add .dockerignore

Symlink to .gitignore.

electrum/tests/regtest.py

@@ -7,9 +7,10 @@ class TestLightning(unittest.TestCase):
 
     @staticmethod
     def run_shell(args, timeout=30):
-        process = subprocess.Popen(['electrum/tests/regtest/regtest.sh'] + args, stderr=subprocess.STDOUT, stdout=subprocess.PIPE)
-        for line in iter(process.stdout.readline, b''):
-            sys.stdout.write(line.decode(sys.stdout.encoding))
+        process = subprocess.Popen(['electrum/tests/regtest/regtest.sh'] + args, stderr=subprocess.STDOUT, stdout=subprocess.PIPE, universal_newlines=True)
+        for line in iter(process.stdout.readline, ''):
+            sys.stdout.write(line)
+            sys.stdout.flush()
         process.wait(timeout=timeout)
         process.stdout.close()
         assert process.returncode == 0