housekeeping: Upgraded Xamarin.Forms to v4.3

[RLittlesII]

What kind of change does this PR introduce?

Upgrades Xamarin.Forms to v4.3

What is the current behavior?

Xamarin.Forms references to v4.2

What is the new behavior?

Xamarin.Forms references to v4.3

What might this PR break?

ReactiveUI.XamForms
ReactiveUI.Events.XamForms

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[glennawatson]

/azp rebuild

[azure-pipelines]

Command 'rebuild' is not supported by Azure Pipelines.

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[glennawatson]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[glennawatson]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #2278 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2278   +/-   ##
=======================================
  Coverage   55.32%   55.32%           
=======================================
  Files         113      113           
  Lines        4334     4334           
  Branches      660      660           
=======================================
  Hits         2398     2398           
  Misses       1771     1771           
  Partials      165      165

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