Requests-cache

[See all issues and PRs for 0.8](https://github.com/requests-cache/requests-cache/milestone/3?closed=1)

🕗 **Expiration & headers:**
* Add support for conditional requests and cache validation using:
* `ETag` + `If-None-Match` headers
* `Last-Modified` + `If-Modified-Since` headers
* `304 Not Modified` responses
* If a cached response is expired but contains a validator, a conditional request will by sent, and a new response will be cached and returned only if the remote content has not changed

💾 **Backends:**
* **Filesystem:**
* Add `FileCache.cache_dir` wrapper property
* Add `FileCache.paths()` method
* Add `use_cache_dir` option to use platform-specific user cache directory
* Return `pathlib.Path` objects for all file paths
* Use shorter hashes for file names
* **SQLite:**
* Add `SQLiteCache.db_path` wrapper property
* Add `use_memory` option and support for in-memory databases
* Add `use_cache_dir` option to use platform-specific user cache directory
* Return `pathlib.Path` objects for all file paths

🚀 **Performance:**
* Use `cattrs` by default for optimized serialization
* Slightly reduce size of serialized responses

↔️ **Request matching:**
* Allow `create_key()` to optionally accept parameters for `requests.Request` instead of a request object
* Allow `match_headers` to optionally accept a list of specific headers to match
* Add support for custom cache key callbacks with `key_fn` parameter
* By default use blake2 instead of sha256 for generating cache keys

ℹ️ **Cache convenience methods:**
* Add `BaseCache.update()` method as a shortcut for exporting to a different cache instance
* Allow `BaseCache.has_url()` and `delete_url()` to optionally take parameters for `requests.Request` instead of just a URL

📦 **Dependencies:**
* Add `appdirs` as a dependency for easier cross-platform usage of user cache directories
* Update `cattrs` from optional to required dependency
* Update `itsdangerous` from required to optional (but recommended) dependency
* Require `requests` 2.22+ and `urllib3` 1.25.5+

⚠️ **Backwards-compatible API changes:**

The following changes are meant to make certain behaviors more obvious for new users, without breaking existing usage:
* For consistency with `Cache-Control: stale-if-error`, rename `old_data_on_error` to `stale_if_error`
* Going forward, any new options based on a standard HTTP caching feature will be named after that feature
* For clarity about matching behavior, rename `include_get_headers` to `match_headers`
* References in the docs to cache keys and related behavior are now described as 'request matching'
* For consistency with other backends, rename SQLite backend classes: `backends.sqlite.Db*` -> `SQLiteCache`, `SQLiteDict`, `SQLitePickleDict`
* Add aliases for all previous parameter/class names for backwards-compatibility

⚠️ **Deprecations & removals:**
* Drop support for python 3.6
* Remove deprecated `core` module
* Remove deprecated `BaseCache.remove_old_entries()` method

-----

* Fix incorrect location of `redirects.sqlite` when using filesystem backend
* Fix issue in which `redirects.sqlite` would get included in response paths with filesystem backend
* Add aliases for forwards-compatibility with 0.8+
* Backport fixes from 0.8.1

* Fix an issue with httpdate strings from `Expires` headers not getting converted to UTC
* Fix a packaging issue with extra files added to top-level wheel directory
* Fix some issues with parallelizing tests using pytest-xdist

* SQLite backend:
* Update `DbCache.clear()` to succeed even if the database is corrupted
* Update `DbDict.bulk_delete()` to split the operation into multiple statements to support deleting more items than SQLite's variable limit (999)
* Filesystem backend:
* When using JSON serializer, pretty-print JSON by default
* Add an appropriate file extension to cache files (`.json`, `.yaml`, `.pkl`, etc.) by default; can be overridden or disabled with the `extension` parameter.
* Add a `BaseCache.delete_urls()` method to bulk delete multiple responses from the cache based on
request URL

* Add support for `Response.next` (to get the next request in a redirect chain) when 302 responses are cached directly
* Add a `CachedResponse.cache_key` attribute
* Make `CachedResponse` a non-slotted class to allow client code to set arbitrary attributes on it

* Fix a bug in which Cache-Control headers would be used unexpectedly