🐛 Fix TensorFlow (and h3py) dependency

TF 2.5 works for Python 3.6 + 3.9. Fixes #66.

requirements.txt

@@ -1,5 +1,4 @@
-h5py < 3  # XXX tensorflow 2.4.0rc3 requires h5py~=2.10.0, but you'll have h5py 3.1.0 which is incompatible.
+tensorflow >= 2.5.0
-tensorflow >= 2.3.0rc2, < 2.5.0 # 2.5 depends on h5py ~= 3.1 (incompatible model files)
 calamari-ocr == 1.0.*
 setuptools >= 41.0.0  # tensorboard depends on this, but why do we get an error at runtime?
 click