[bugfix] Set network value to 'mainnet' in blockfrost if only mainnet api url is provided

[cffls]

Fixes #349

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.56%. Comparing base (20e8ed7) to head (a580dff).

Files	Patch %	Lines
pycardano/backend/blockfrost.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
- Coverage   83.61%   83.56%   -0.05%     
==========================================
  Files          29       29              
  Lines        3722     3724       +2     
  Branches      939      940       +1     
==========================================
  Hits         3112     3112              
- Misses        445      447       +2     
  Partials      165      165              

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