Fix ImportError: cannot import name '_to_bytes' error

[gene1wood]

This constrains the requests-cache package to a version older than 0.6.0 in which the _to_bytes method was removed. This method is used in tvdb_api. Once use of the _to_bytes methods is removed from tvdb_api, this version contraint can be removed.

Fixes #92

[codecov[bot]]

Codecov Report

Merging #93 (d2347f5) into master (92e8c7c) will decrease coverage by 0.20%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   82.42%   82.22%   -0.21%     
==========================================
  Files           1        1              
  Lines         495      495              
==========================================
- Hits          408      407       -1     
- Misses         87       88       +1     

Impacted Files	Coverage Δ
tvdb_api.py	82.22% <0.00%> (-0.21%)	:arrow_down:

[dbr]

Thanks for this! Shall make a new patch release with this soon