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)

capnp/lib/capnp.pyx

@@ -4070,6 +4070,17 @@ def add_import_hook(additional_paths=[]):
     if _importer is not None:
         remove_import_hook()
 
+    # Automatically include the system and built-in capnp paths
+    # Highest priority at position 0
+    extra_paths = [
+        _os.path.join(_os.path.dirname(__file__), '..'), # Built-in (only used if bundled)
+        '/usr/local/include/capnp', # Common macOS brew location
+        '/usr/include/capnp', # Common posix location
+    ]
+    for path in extra_paths:
+        if _os.path.isdir(path):
+            additional_paths.append(path)
+
     _importer = _Importer(additional_paths)
     _sys.meta_path.append(_importer)