* socket_helper.transient_internet() no longer imports nntplib to
catch nntplib.NNTPTemporaryError.
* ssltests.py no longer runs test_nntplib.
* "make quicktest" no longer runs test_nntplib.
* WASM: remove nntplib from OMIT_NETWORKING_FILES.
* Remove mentions to nntplib in the email documentation.
requires them. Disable executable bits and shebang lines in test and
benchmark files in order to prevent using a random system python, and in
source files of modules which don't provide command line interface. Fixed
shebang lines in the unittestgui and checkpip scripts.