Work around a file descriptor issue in cheroot.

There is an issue that was introduced in 8.1.0 that throws an error when
file descriptor numbers are large (see
cherrypy/cheroot#249).  We can't pin cheroot
to an older version, as that breaks support for modern python versions.

It isn't clear if limiting the size of our cache will really help -- the
cheroot issue implies that it is leaking file descriptors, so this may
not do anything useful.

large_image/cache_util/cache.py

@@ -39,6 +39,10 @@
         SoftNoFile, HardNoFile = resource.getrlimit(resource.RLIMIT_NOFILE)
         resource.setrlimit(resource.RLIMIT_NOFILE, (HardNoFile, HardNoFile))
         SoftNoFile, HardNoFile = resource.getrlimit(resource.RLIMIT_NOFILE)
+        # When run under cheroot, cheroot can fail if the file descriptor is
+        # greater than 1024 (see github.com/cherrypy/cheroot/issues/249), so
+        # limit this further
+        SoftNoFile = min(SoftNoFile, 1024)
         # Reserve some file handles for general use, and expect that tile
         # sources could use many handles each.  This is conservative, since
         # running out of file handles breaks the program in general.