BUG: Fix typos in conversion_utils

Unparenthesized logical expressions in if statements caused clang
compilation failure on OSX.
BUG: fix minor memory leak in memoryview failure case

backport of 2f8d33bc755b869639e38e0482c0abda956578cc
BUG: fix build with single-threaded python

Some symbols are not defined leading to:
ImportError Undefined symbol: PyGILState_Release

Closes gh-3255
DOC: add versionadded:: 1.8.0 to ufunc.at docstring
Merge pull request #4226 from juliantaylor/backports-1.8

Backports more fixes to 1.8