Support WATCHFILES_FORCE_POLLING env var

[samuelcolvin]

Partial fix / work around for #167

[codecov]

Codecov Report

Merging #170 (e5bf029) into main (57fbbe1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #170   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          375       382    +7     
  Branches        79        80    +1     
=========================================
+ Hits           375       382    +7     

Impacted Files	Coverage Δ
watchfiles/main.py	100.00% <100.00%> (ø)

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