fix bug when calling propertynames on a different thread with gil handled properly

[cjdoris]

the fix is to do the propertynames logic directly on the current thread when it already holds the gil, rather than using on_main_thread

fixes #751

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	d0c0662...	main / d0c0662...
basic/@py/pydict/init	0.237 ± 0.035 ms	0.239 ± 0.036 ms	0.992 ± 0.21
basic/@py/pydict/pydel	0.239 ± 0.023 ms	0.24 ± 0.024 ms	0.996 ± 0.14
basic/julia/pydict/init	0.209 ± 0.046 ms	0.208 ± 0.048 ms	1 ± 0.32
basic/julia/pydict/pydel	0.219 ± 0.027 ms	0.224 ± 0.027 ms	0.981 ± 0.17
gc/full	0.632 ± 0.004 s	0.633 ± 0.0022 s	0.998 ± 0.0072
time_to_load	2 ± 0.039 s	2.02 ± 0.04 s	0.988 ± 0.027

Memory benchmarks

	main	d0c0662...	main / d0c0662...
basic/@py/pydict/init	5.01 k allocs: 0.0764 MB	5.01 k allocs: 0.0764 MB	1
basic/@py/pydict/pydel	5.01 k allocs: 0.0764 MB	5.01 k allocs: 0.0764 MB	1
basic/julia/pydict/init	4.01 k allocs: 0.0612 MB	4.01 k allocs: 0.0612 MB	1
basic/julia/pydict/pydel	4.01 k allocs: 0.0612 MB	4.01 k allocs: 0.0612 MB	1
gc/full	0 allocs: 0 B	0 allocs: 0 B
time_to_load	0.143 k allocs: 10.6 kB	0.143 k allocs: 10.6 kB	1