pdb: restore SIGINT handler in sigint_handler already

[1f7c169c-a6ad-45ae-b614-0d40d962c776]

BPO	36667
Nosy	@blueyed
PRs	python/cpython#12880

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-bug', 'library', '3.9'] title = 'pdb: restore SIGINT handler in sigint_handler already' updated_at = user = 'https://github.com/blueyed' ``` bugs.python.org fields: ```python activity = actor = 'blueyed' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = creator = 'blueyed' dependencies = [] files = [] hgrepos = [] issue_num = 36667 keywords = ['patch'] message_count = 1.0 messages = ['340539'] nosy_count = 1.0 nosy_names = ['blueyed'] pr_nums = ['12880'] priority = 'normal' resolution = None stage = 'patch review' status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue36667' versions = ['Python 3.9'] ```

[1f7c169c-a6ad-45ae-b614-0d40d962c776]

Without this, and additional SIGINT while waiting for the next statement (e.g. during time.sleep) will stop at sigint_handler.

With this patch:

> …/t-pdb-sigint-in-sleep.py(10)<module>()
-> sleep()
(Pdb) c
^C
Program interrupted. (Use 'cont' to resume).
^CKeyboardInterrupt
> …/t-pdb-sigint-in-sleep.py(6)sleep()
-> time.sleep(10)
(Pdb)

Without this patch:

> …/t-pdb-sigint-in-sleep.py(10)<module>()
-> sleep()
(Pdb) c
^C
Program interrupted. (Use 'cont' to resume).
^C--Call--
> …/cpython/Lib/pdb.py(188)sigint_handler()
-> def sigint_handler(self, signum, frame):
(Pdb)

This was changed / regressed in https://github.com/python/cpython/commit/10e54aeaa234f2806b367c66e3fb4ac6568b39f6 (3.5.3rc1?), when it was moved while fixing bpo-20766.