Work with Windows’ different epoch

[adamchainz]

Fixes #89.

[codecov-io]

Codecov Report

Merging #98 (1d02532) into master (71cf08a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #98   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          209       213    +4     
  Branches        38        38           
=========================================
+ Hits           209       213    +4     

Impacted Files	Coverage Δ
src/time_machine.py	100.00% <100.00%> (ø)

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