Use new headers API from Playwright 1.15

[elacuesta]

See https://playwright.dev/python/docs/release-notes#version-115

Tasks:

• Use new headers API
• Deprecate scrapy_playwright.headers.use_playwright_headers in favor of setting PLAYWRIGHT_PROCESS_REQUEST_HEADERS=None

[codecov]

Codecov Report

Merging #93 (433cdb5) into master (a632118) will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #93      +/-   ##
===========================================
+ Coverage   99.64%   100.00%   +0.35%     
===========================================
  Files           4         4              
  Lines         285       297      +12     
===========================================
+ Hits          284       297      +13     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
scrapy_playwright/handler.py	100.00% <100.00%> (ø)
scrapy_playwright/headers.py	100.00% <100.00%> (+6.66%)	⬆️

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