Updating to capnproto v0.8.0

- Includes some test stabilization
- Fixes manylinux2010 build issues (linker flag order due to old gcc)
- More rigorous python setup.py clean
- Requires capnproto v0.8.0 or greater
- Including system libcapnp include path for import (e.g. import
  stream_capnp)
- Bundle libcapnp .capnp files when not using system libcapnp
- Removing more distutils usage. Now using pkg-config to determine the
  system version of libcapnp (mainly for Linux, but should work on macOS
  with brew)
- Removed dead code

Resolves issues #215 #216 #217
Lots of fixes for Issue #218 (all sorts of retry methods needed for
GitHub Actions)

test/test_load.py

@@ -111,3 +111,9 @@ def test_remove_import_hook():
 
     with pytest.raises(ImportError):
         import addressbook_capnp  # noqa: F401
+
+
+def test_bundled_import_hook():
+    # stream.capnp should be bundled, or provided by the system capnproto
+    capnp.add_import_hook()
+    import stream_capnp