Change `request_iteration_count` to use a GET request

[TillerBurr]

This is the fix suggested in https://github.com/konomae/lastpass-python/issues/44#issuecomment-847096643. It was tested with my personal account and it works well.

Whilst testing, I also found that changing the request to /iterations.php to a GET is not enough. The User-Agent in the header must also be changed from python/requests to something else. It doesn't matter what it's changed to. For this repo, this was already fixed in commit fad6e0cea014796ed96235ae85c1fa7a42c54860.

As noted in https://github.com/lastpass/lastpass-cli/issues/604, one workaround is to change the iterations to 100100 by performing the following:

1. Open Account Settings in your browser (Open My Vault → Account Settings)
2. Press Show Advanced Settings
3. Set General → Security → Password Iterations to exactly 100100

LastPass will ask for your Master password and re-encrypt your vault.

Closes https://github.com/konomae/lastpass-python/issues/44

[codecov[bot]]

Codecov Report

Merging #45 (05518e8) into master (41af73a) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 05518e8 differs from pull request most recent head 5c3ddb3. Consider uploading reports for the commit 5c3ddb3 to get more accurate results

@@           Coverage Diff           @@
##           master      #45   +/-   ##
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  Coverage   72.72%   72.72%           
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  Files          10       10           
  Lines         385      385           
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  Hits          280      280           
  Misses        105      105           

Impacted Files	Coverage Δ
lastpass/fetcher.py	60.97% <100.00%> (ø)

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