Refactor CachedSession to be usable as a mixin class

[JWCook]

Closes:

• Support using requests-cache with other libraries that extend requests.Session #158
• remove_expired_responses() not working when global hook not installed #163
• Compatibility with requests_html.HTMLSession #149
• Using with OriginalSession from requests subclass  #146
• Detect if requests-cache is active or not #144
• Add compatibility with requests-mock #87

See notes on those issues for more background info. See examples below for usage info.
Examples have also been added to the docs.

Example with requests-html (#149)

import requests
from requests_cache import CacheMixin
from requests_html import HTMLSession

class CachedHTMLSession(CacheMixin, HTMLSession):
    """Session with features from both CachedSession and HTMLSession"""

session = CachedHTMLSession()
r = session.get("https://github.com/")
print(r.from_cache, r.html.links)

Or, using the monkey-patch method:

from requests_cache import install_cache

install_cache(session_factory=CachedHTMLSession)
r = requests.get("https://github.com/")
print(r.from_cache, r.html.links)

Note that this usage would be more ideal if requests-html could also be used as a mixin.

Example with internetarchive (#158)

Pretty much the same as above:

import requests
from requests_cache import CacheMixin
from internetarchive.session import ArchiveSession

class CachedArchiveSession(CacheMixin, ArchiveSession):
    """Session with features from both CachedSession and ArchiveSession"""

Example with requests-mock (#87)

import requests
from requests_mock import Mocker
from requests_cache import CachedSession

session = CachedSession()
with Mocker(session=session) as m:
    m.get('http://test.com', text='mock_response')
    response = session.get('http://test.com')
    print(response.text)

[coveralls]

Coverage increased (+1.2%) to 85.644% when pulling 0394a90 on JWCook:cache-mixin into bfe214e on reclosedev:master.

[JWCook]

@kousu @jeremydouglass If you get the chance, test those changes out for me and let me know if that does the trick. Changes are in the current master branch, not yet released.

[jeremydouglass]

Hi @JWCook, this is extremely exciting, and apologies for not seeing this earlier. I'll try to take a look shortly based on my original use case from Jan 2020 in #158

[JWCook]

Thanks! Any feedback on that would be appreciated.