dap-python: Support attach to an existing process #520

Job	Run time
test (ubuntu-latest, 27.2, false)	1m 32s
test (ubuntu-latest, 28.2, false)	1m 44s
test (ubuntu-latest, 29.1, false)	1m 30s
test (macos-latest, 27.2, false)	2m 56s
test (macos-latest, 28.2, false)	3m 16s
test (macos-latest, 29.1, false)	2m 49s
test (windows-latest, 27.2, false)	2m 24s
test (windows-latest, 28.2, false)	1m 59s
test (windows-latest, 29.1, false)	2m 2s
test (ubuntu-latest, snapshot, true)	1m 36s
test (macos-latest, snapshot, true)	3m 24s
test (windows-latest, snapshot, true)	39s
	25m 51s

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