bpo-29602: fix signed zero handling in complex constructor

[mdickinson]

Backport of #203 to branch for 3.5.

[codecov]

Codecov Report

Merging #205 into 3.5 will increase coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## 3.5 #205 +/- ##
==========================================
+ Coverage 82.41% 82.41% +<.01% 
==========================================
Files 1404 1404 Lines 341433 341454 +21 ==========================================
+ Hits 281390 281411 +21 
Misses 60043 60043

Impacted Files	Coverage Δ
Lib/test/test_complex.py	98.41% <100%> (+0.06%)	✅
Lib/test/test_select.py	87.69% <ø> (-1.54%)	❌
Lib/threading.py	81.73% <ø> (-0.87%)	❌
Lib/test/test_poll.py	92.02% <ø> (-0.62%)	❌
Lib/test/lock_tests.py	86.17% <ø> (-0.59%)	❌
Lib/test/test_thread.py	66.32% <ø> (-0.52%)	❌
Lib/nntplib.py	81.21% <ø> (-0.36%)	❌
Lib/test/test_buffer.py	96.53% <ø> (-0.18%)	❌
Lib/test/test_weakref.py	96.58% <ø> (-0.16%)	❌
Lib/pydoc.py	62.05% <ø> (-0.07%)	❌
... and 9 more

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