Contextvars-extras

Feature
* Remove "under development" warnings, the code is stable enough ([`6e5518b`](https://github.com/vdmit11/contextvars-extras/commit/6e5518bd4be621f4f481297225970a05bb5abd7f))

Fix
* ContextVarsRegistry.keys() triggers deferred_default ([`fc17c59`](https://github.com/vdmit11/contextvars-extras/commit/fc17c59c3518efa4190cef5a1ce7f02088df210a))

Feature
* New method: `ContextVarExt.is_gettable` ([`77ff938`](https://github.com/vdmit11/contextvars-extras/commit/77ff9382bb72026905d86f0872e95316ee85a255))
* New flag: `ContextVarExt.default_is_set` ([`296fc9f`](https://github.com/vdmit11/contextvars-extras/commit/296fc9fefc00ce56020fd56556e98f4317987fb1))

Feature
* Allow setting properties using `with registry(var=value)` ([`0a02cf6`](https://github.com/vdmit11/contextvars-extras/commit/0a02cf6d0f263f743def3c8c66bf9e20302930c2))

Fix
* Missing type stub for ContextVarExt.deferred_default attribute ([`b088ea9`](https://github.com/vdmit11/contextvars-extras/commit/b088ea94fd9eaafdf802c8011e6224459d9b1958))

Fix
* Wrong exception type thrown by ContextVarsRegistry.__delitem__ ([`876d342`](https://github.com/vdmit11/contextvars-extras/commit/876d3421519b2a8fca71eb1c0f9c596a3e816b49))

Documentation
* Change order of classes to put ContextVarExt to the top ([`a115a64`](https://github.com/vdmit11/contextvars-extras/commit/a115a64e67ccd554b7fdf2a204f3a740b83b5981))