Dclab

- fix: support "image_bg" feature for DCOR data
- ref: replace formats with f-strings in check.py

- enh: reduce computation time of file hashes in the cli submodule
(and thus all other software downstream) by switching from a full
SHA256 hash to a 5MB md5 hash; this speeds up operations on
slow network shares
- ref: minor change when installing R packages
(rlme tests are broken on Windows currently)
- ref: use np.float32 for principal inertia ratio computation
(compatibility with OpenCV)

- feat: pull DCOR access token management from downstream DCOR-Aid
- feat: allow to set alternate DCOR server certificate bundles by
appending paths `dclab.rtdc_dataset.fmt_dcor.DCOR_CERTS_SEARCH_PATHS`
- enh: allow to skip checks and by default use the innate features
during export of an RTDCBase
- setup: remove appveyor build pipeline
- ref: cleanup fmt_dcor

- setup: bump numpy from 1.17.0 to 1.21.0
- setup: bump scipy from 0.14.0 to 1.8.0
- setup: drop support for Python 3.7

- fix: regression since 0.39.0; contours with wrong index where not
repaired
- ref: Python distutils library is deprecated
- ref: some scipy namespaces are deprecated

- enh: allow to specify unique_id of PolygonFilter when loading
from file