v2.6.0 — opt-in session request defaults

Opt-in session request defaults

Version 2.6.0 adds Session.request_defaults and
AsyncSession.request_defaults for reusable headers and string query pairs.

• Defaults are disabled initially. Assign a headers/params configuration to
  opt in; assign None or {} to clear it. No constructor changes are required.
• Configuration is validated and copied atomically; the getter returns a
  detached snapshot. Header names merge case-insensitively, and repeated query
  pairs retain order. Explicit request values override configured values.
• Empty supported inputs and raw queries retain their native bypass behavior.
  Defaults merge only before initial preparation, not again on hooks, retries,
  redirects or sends of an already prepared request.
• Authorization, proxy authorization, Cookies, Host and body-framing headers
  remain in their existing APIs and cannot be configured as defaults.
• Documentation, executable local examples, public typing and 138 focused
  unit/peer cases cover both clients, prepared snapshots and policy boundaries.

Python >=3.7 remains supported. Unconfigured sessions keep their existing
behavior; legacy Session observation properties do not become request defaults.
TLS/H2 engines, browser-profile support and existing timeout/proxy contracts
are unchanged. This release does not claim full Requests or browser parity.

See the configuration guide
and changelog.