Correct SSL type annotations

#992
https://docs.python.org/3/library/ssl.html#ssl.SSLContext.load_cert_chain

- `certfile` is a required positional argument when running
  `SSLContext.load_cert_chain`, so annotating as `Optional` (which
  allows `None`) would not be ideal. Path-like objects are acceptable,
  so after `from pathlib import Path`, the annotation is
  `certfile: Union[Path, str]`.
- `if self.is_ssl and self.ssl_certfile` will help ensure that the
  `self.ssl_certfile` required positional argument is present.

Author: br3ndonland

uvicorn/config.py

@@ -108,7 +108,7 @@
 
 
 def create_ssl_context(
-    certfile: Optional[str],
+    certfile: Union[Path, str],
     keyfile: Optional[str],
     password: Optional[str],
     ssl_version: int,
@@ -161,7 +161,7 @@ def __init__(
         timeout_notify: int = 30,
         callback_notify: Callable[..., None] = None,
         ssl_keyfile: Optional[str] = None,
-        ssl_certfile: Optional[str] = None,
+        ssl_certfile: Optional[Union[Path, str]] = None,
         ssl_keyfile_password: Optional[str] = None,
         ssl_version: int = SSL_PROTOCOL_VERSION,
         ssl_cert_reqs: int = ssl.CERT_NONE,
@@ -286,9 +286,8 @@ def configure_logging(self) -> None:
     def load(self) -> None:
         assert not self.loaded
 
-        self.ssl: Optional[ssl.SSLContext]
-        if self.is_ssl:
-            self.ssl = create_ssl_context(
+        if self.is_ssl and self.ssl_certfile:
+            self.ssl: Optional[ssl.SSLContext] = create_ssl_context(
                 keyfile=self.ssl_keyfile,
                 certfile=self.ssl_certfile,
                 password=self.ssl_keyfile_password,