Require pydantic<2.0 unless it's supported (#573)

Pydantic 2.0 has been released yesterday and is not compatible with the current code.

requirements.txt

@@ -12,4 +12,4 @@ configargparse>=1.2.3
 multiaddr>=0.0.9
 pymultihash>=0.8.2
 cryptography>=3.4.6
-pydantic>=1.8.1
+pydantic>=1.8.1,<2.0