SPOT v1.5.3

• Update requirement to Ginga v7.6.0 (needed for the SkyCam differential
  image to stay in place when zoomed in)
• Update the optional "pg" (web browser) requirements to pgwidgets-js
  and pgwidgets-python v0.4.3
• SkyCam: reworked the differential sky image to make it useful for
  spotting clouds. Frames are median filtered to remove stars and noise,
  scaled to cancel camera exposure changes, optionally derotated with the
  sky so that stars cancel, and shown in units of the image noise. A
  cloud cover percentage is reported, with an adjustable threshold, and
  there is a choice of color maps that keep the target overlays readable.
  Fixed the old subtraction, which wrapped around for color images and
  dropped areas that got darker for monochrome ones.
• SkyCam: the sky image now uses the viewer's color map and cut levels,
  so they can be adjusted (the normal and differential image each keep
  their own); the image is no longer auto-leveled on every download.
• SkyCam: fixed the sky image being off-center (misaligned with the polar
  plot) for cameras whose sky circle runs off the edge of the frame, such
  as GTC.
• SkyCam: the cloud cover shows "Twilight" or "Sun up" instead of a
  value while the Sun is above -12 degrees at the camera.
• SkyCam: at night a white arrow from the zenith shows the direction and
  speed (degrees per minute) of the clouds, and targets in or near clouds
  are shown in orange-red in the Targets plugin ("Show Cloud Motion" and
  "Flag Clouded Targets"). The motion is found by correlating the last two
  differences between images, which removes fixed features such as domes.
• Targets: the rotating target colors are now bright colors that show up
  on the sky camera images and are distinct from the selected, tagged and
  clouded target colors (green, yellow, orange, plum, chartreuse, pink).
• SkyCam: a "History" tab plots the cloud cover over the last 12 hours
  (the controls moved to a "Camera" tab). The cloud cover is now
  measured for every new image, whether or not the differential image is
  shown.
• SkyCam: image processing now runs off the GUI thread, so large camera
  images no longer make the interface pause.
• SkyCam: animated images (e.g. GIF loops) are supported; the last frame
  is used unless a "frame" is given in skycams.yml. The CFHT Nana ao
  cameras, whose still images are no longer published, now use their
  GIF loops (and the Mid-IR camera has new geometry).
• skycams.yml: cameras can give their own "latitude" and "longitude",
  used to derotate the sky and to tell when it is night at the camera;
  the GTC camera URL now uses https.