MNT: trigger readthedocs on py 3.7

ENH: make wradlib.io capable of consuming file-like objects (#469)

* ENH: make hdf, iris, netcdf, radolan and rainbow readers consume file-like objects, adapt tests
* ENH: make odim reader work with file-like objects, adapt tests

FIX: adapt to new string behaviour in h5py >= 3.0 (#468)

FIX: add capability to decode old DX header, add test (#467)

MNT: update README.md [skip ci]

MNT: add Python 3.9 builds to all CI (#463)

* MNT: add Python 3.9 builds to all CI
* MNT: reduce build matrices

MNT: use micromamba/mamba on Azure, mamba on Appveyor (#465)

* use micromamba on Azure for Linux and OSX builds
* use conda/mamba on Azure for Windows builds
* use conda/mamba on Appveyor
* only use matplotlib-base to not pull in PyQt/Qt
* update isort to latest release...

MNT: update micromamba in Travis CI (#464)

Simplify dimension angle handling ODIM/GAMIC (#462)

*simplify azimuth/elevation handling in ODIM/GAMIC reader

MNT: Use micromamba/mamba for bootstrapping conda environment (#452)

MNT: Rel 1.8.0

MNT: remove stale print-statement, initialize numpy-array as float

MNT: add `WN` and `YW` products to radolan to xarray converter

MNT: add `mode`-kwarg to `h5py.File()` (deprecated)

MNT: replace deprecated xarray `.drop` with `.drop_vars`

MNT: add `dtype=object` when creating jagged/ragged numpy arrays (deprecated)

MNT: remove deprecated `squeeze=None` in georef.polar.py

MNT: sort requirements.txt

MNT: remove testrunner.py (not used anymore)

MNT: do not run CI tests in parallel