fix working with newer requests-cache versions

[vapier]

Just inline the simple internal symbols that were being imported
from requests-cache (since it no longer exports them).

Fixes #94.

[dbr]

Thanks for this! Merged, and shall try and get this into a tagged release as soon as possible

[PumpkinCheshire]

Sorry I think the error still exist:

ImportError: cannot import name '_to_bytes' from 'requests_cache.backends.base' (/usr/lib/python3.9/site-packages/requests_cache/backends/base.py)

[almightiest]

I was running into this issue with python 3.8 and tvdb-api 3.1.0 with requests-cache 0.9.4.

I didn't realize 3.1.0 doesn't have the change. Looks like a new version still needs to be released, my bad on missing that.

Workaround: I was able to clone the repo, and just do a pip install -U . from the local folder to upgrade, which lets me keep the newer requests-cache version.

pip install -U requests-cache
...
tvdb-api 3.1.0 requires requests-cache<0.6.0, but you have requests-cache 0.9.4 which is incompatible.

invoking tvdb_api throws:

Traceback (most recent call last):
  File "/usr/local/bin/thetvdb.py", line 28, in <module>
    import tvdb_api
  File "/home/bleh/.local/lib/python3.8/site-packages/tvdb_api.py", line 34, in <module>
    from requests_cache.backends.base import _to_bytes, _DEFAULT_HEADERS
ImportError: cannot import name '_to_bytes' from 'requests_cache.backends.base' (/home/bleh/.local/lib/python3.8/site-packages/requests_cache/backends/base.py)

[frank42hh]

Hmmm, after reading more issues here, I assume that the patch
637b2d2
never got it into the release, since 3.1.0 still tries to import _to_bytes from requests-cache.