Implement SemLock with eventfd where available (3.10+/Linux)

[arcivanov]

Otherwise SemLock is semaphore-based and blocks when RLock

eventfd-based solution doesn't work with forkserver

fixes #16

[coveralls]

Pull Request Test Coverage Report for Build 2142545301

• 86 of 97 (88.66%) changed or added relevant lines in 1 file are covered.
• 14 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-2.8%) to 75.817%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/python/geventmp/_mp/3/_mp_synchronize.py	86	97	88.66%

Files with Coverage Reduction	New Missed Lines	%
src/main/python/geventmp/monkey.py	1	74.75%
src/main/python/geventmp/_mp/3/_mp_connection.py	1	60.98%
src/main/python/geventmp/_mp/3/_mp_synchronize.py	1	86.49%
src/main/python/geventmp/_mp/3/_mp_forkserver.py	11	53.95%

Totals
Change from base Build 1826656772:	-2.8%
Covered Lines:	280
Relevant Lines:	345

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2142563301

• 88 of 97 (90.72%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+2.1%) to 80.785%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/python/geventmp/_mp/3/_mp_synchronize.py	88	97	90.72%

Files with Coverage Reduction	New Missed Lines	%
src/main/python/geventmp/_mp/3/_mp_synchronize.py	1	87.84%

Totals
Change from base Build 1826656772:	2.1%
Covered Lines:	314
Relevant Lines:	365

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💛 - Coveralls