Add exceptions and restructure _fetch_json

[rklomp]

Add exceptions. All methods should return a valid value. If not, an exception should be raise. These exceptions can be handled in HA to more easily provide feedback when things go wrong during setup or polling of data.

• For example raise ConfigEntryNotReady if unable to connect during entry setup

• Makes it easier to create a reauth flow in the future

Also _fetch_json has been renamed to _post_json and new method _get_json was added. These both call _request_json.

• timeouts are now handled by ClientTimeout instead of async_timeout (dependency removed)
• retries using for _ in range(3) have been removed. It just fails and the DataCoordinator in HA will handle the retries.

[codecov[bot]]

Codecov Report

Merging #68 (41fb89b) into master (9953118) will increase coverage by 2.22%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   93.47%   95.69%   +2.22%     
==========================================
  Files           5        6       +1     
  Lines         383      395      +12     
==========================================
+ Hits          358      378      +20     
+ Misses         25       17       -8     

Impacted Files	Coverage Δ
pysma/__init__.py	90.47% <100.00%> (+5.38%)	:arrow_up:
pysma/const.py	100.00% <100.00%> (ø)
pysma/exceptions.py	100.00% <100.00%> (ø)

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