test.regrtest: detect leak of Windows handles

[vstinner]

On "Refleaks" buildbot jobs, test.regrtest checks for leaks of references, memory blocks and file descriptors.

I propose to check also for leaks of Windows handles by calling GetProcessHandleCount() with GetCurrentProcess().

Linked PRs

• gh-154137: Check for Windows handle leaks in regrtest #154140
• gh-154137: Fix handle leak in test_winapi #154201
• [3.15] gh-154137: Fix handle leak in test_winapi (GH-154201) #154202
• [3.14] gh-154137: Fix handle leak in test_winapi (GH-154201) #154203
• [3.13] gh-154137: Fix handle leak in test_winapi (GH-154201) #154204