Add reference user agent for ebay crawler

[codders]

EBay appears to filter some user agents that correspond to scripts. Since the URLs we access don't seem to be excluded by robots.txt, one assumes this is over-zealous DoS attack protection. Use a browser User Agent to avoid the filter.

[codecov-commenter]

Codecov Report

Merging #8 into master will increase coverage by 0.09%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   59.05%   59.15%   +0.09%     
==========================================
  Files           7        7              
  Lines         425      426       +1     
  Branches       50       50              
==========================================
+ Hits          251      252       +1     
  Misses        159      159              
  Partials       15       15              

Impacted Files	Coverage Δ
flathunter/crawl_ebaykleinanzeigen.py	61.42% <66.66%> (+0.55%)	⬆️

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[mordax7]

LGTM