Defer pygments and pdb imports (9% speedup)

[giampaolo]

Hello,
pytest currently imports pygments and pdb on every run, even though they are only needed in specific cases:

• pygments is only used when there is a failure, for the summary shown at the end, and only when terminal colors are enabled.
• pdb is only used with the --pdb CLI option, and only if a test fails.

Note: pdb became noticeably more expensive to import in Python 3.14, as it now pulls in socket, asyncio, and selectors modules, which together bring in roughly 40 additional modules.

Using this patched pytest version against the psutil test suite (839 tests) with --plugins-disabled and --collect-only, I see about a 9% speedup:

~/svn/pytest {speedup}$ python3 bench_real.py
upstream/main   378.8ms
working tree    343.8ms
-> +35.0ms (+9.2%)

bench_real.py