New version of pcsc-lite: 1.8.21

I just released a new version of pcsc-lite 1.8.21.
pcsc-lite is a Free Software implementation of the PC/SC (or WinSCard) API for Unix systems.

Changes:
1.8.21: Ludovic Rousseau
20 May 2017

• SCardCancel():
• fix a race condition bug
• fix a possible use after free
• improve Doxygen documentation
• SCardGetStatusChange(): fix a race condition when a reader is removed
• SCardDisconnect(): fix status update for SCARD_UNPOWER_CARD
• pcsc_stringify_error(): use Thread-local storage
• pcsc_stringify_error() now returns a const char *
• SCardControl() may return SCARD_E_INSUFFICIENT_BUFFER when pbRecvBuffer is not big enough to receive the card response.
• Fix compilation error with GCC 4.9
• UnitaryTests: add support of Python 3
• pcsc-spy: add support of Python 3
• Some other minor improvements