Stop patches in the reverse order they were started

[kurtmckee]

Because some functions are patched twice (like os.rename()), stopping the patches in the same order they were started can result in restoration of one of the patches.

For example:

fn = os.rename

# Patch nesting order: 1 then 2
os.rename = patch_1(fn)
os.rename = patch_2(patch_1(fn))

# Unpatch 1 then 2: A still-patched function is restored
os.rename = fn
os.rename = patch_1(fn)

# Unpatch 2 then 1: The original function is restored
os.rename = patch_1(fn)
os.rename = fn

Fixes #365

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.48%. Comparing base (7e6ec66) to head (33f5390).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #370   +/-   ##
=======================================
  Coverage   89.48%   89.48%           
=======================================
  Files          36       36           
  Lines        1674     1674           
  Branches      205      205           
=======================================
  Hits         1498     1498           
  Misses        143      143           
  Partials       33       33           

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[anishathalye]

Thank you!