bpo-25872: Fix KeyError in linecache when multithreaded

[mgraczyk]

Fixes https://bugs.python.org/issue25872

The crash that this fixes occurs when using traceback and other modules from multiple threads.

https://bugs.python.org/issue25872

[the-knights-who-say-ni]

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[csabella]

@mgraczyk, thank you for the contribution, but as the original patch was for Python 2.7, I'm going to close this issue.

[mgraczyk]

@csabella The issue still exists on master. Do I need to open a new bpo and submit this identical PR referencing the new issue?