Add individual expire_after per request. (#177)

* Add individual expire_after per request.

This commit adds a new feature to allow individual expire_after times
per request.
It involves using a custom CachedSession, PerRequestCachedSession,
to be used as the session_factory.

* Replace deprecated asserts in unit tests

Additionally, some sockets are closed even on test failures.
This removes some visual clutter when going through the results.

* Update docs and README for per-request caching

* Remove per_request and integrate behavior into CachedSession

* Make test_return_old_data_on_error more reliable

This is achieved by using a fixed date and mocking the backend calls to datetime as well.

* Adjust per_request cache to new behavior

Also adds HTTPBIN_URL environment variable as in test_cache

* Add tests for the function determining the expiration datetime

* Adjust backend to store expiration dates

This also involves slightly different semantics for deletion, but the behavior stays the same.

* Minor: black and isort

* Update docs for request

Add explanation for expire_after special cases, i.e. 'default' and 'cached' and how precedences are handled

* Replace assert statements with methods

* Remove invalid snippet from README

* Remove invalid sentence from docs.

* Make expires_before timezone-aware once

And combine the checks for expiration

* Simplify complexity by removing 'cached' keyword for expire_after

1 files changed, 1 insertions, 0 deletions

diff --git a/docs/conf.py b/docs/conf.py
index 14fea4f..d152386 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -32,6 +32,7 @@ extensions = [
 
 # Enable automatic links to other projects' Sphinx docs
 intersphinx_mapping = {
+    'python': ('https://docs.python.org/3', None),
     'requests': ('https://requests.readthedocs.io/en/master/', None),
 }