Deprecate this fork and redirect users to the maintained Photometrics PyVCAM release

[Copilot]

Photometrics has now closed the upstream live-mode queue issue and confirmed that the fix from our forked work was applied in their maintained repository. This PR updates this fork to act as a migration shim rather than a source of truth.

• Upstream retirement signal

  • Add an import-time DeprecationWarning in pyvcam that directs consumers to the maintained Photometrics PyPI package.
  • Keep the change minimal and non-invasive: existing imports still work, but they now surface the migration path.

• Docs updated for migration

  • Mark this fork as deprecated in the main README and wrapper docs.
  • Replace source-install guidance with the intended install path:
    • python -m pip install --upgrade PyVCAM
  • Update the cheatsheet to point to the upstream package instead of local setup.py install.

• Focused coverage

  • Add a narrow unit test that verifies importing pyvcam emits the expected deprecation warning.

• Repo hygiene

  • Ignore Python bytecode/cache artifacts so generated files do not get committed during future maintenance.

Example of the new migration signal:

import warnings

with warnings.catch_warnings(record=True) as caught:
    warnings.simplefilter("always")
    import pyvcam

print(caught[0].category.__name__)
print(caught[0].message)
# DeprecationWarning
# This fork of PyVCAM is deprecated. Install the maintained Photometrics release from PyPI instead.