Tweak create_ssl_context defaults. (#1447)

* Version 0.17.0

* create_ssl_config uses trust_env=True default

* Drop CHANGELOG notes, to be included in a version PR instead

Author: lovelydinosaur

httpx/_config.py

@@ -43,7 +43,7 @@ class UnsetType:
 def create_ssl_context(
     cert: CertTypes = None,
     verify: VerifyTypes = True,
-    trust_env: bool = None,
+    trust_env: bool = True,
     http2: bool = False,
 ) -> ssl.SSLContext:
     return SSLConfig(
@@ -63,7 +63,7 @@ def __init__(
         *,
         cert: CertTypes = None,
         verify: VerifyTypes = True,
-        trust_env: bool = None,
+        trust_env: bool = True,
         http2: bool = False,
     ):
         self.cert = cert