Qsymm

+ Additionally allow using strings or sympy expressions in `PointGroupElement`
for the rotation part [gitlab issue
7](https://gitlab.kwant-project.org/qt/qsymm/-/issues/7).
+ Avoid a numerical instability in symmetry finder [gitlab issue
49](https://gitlab.kwant-project.org/qt/qsymm/-/issues/49).
+ Fix compatibility with latest sympy v1.12.

Added

+ integration test: run the tests for Kwant's qsymm module against the current version
of qsymm.

Fixed

+ Avoided using deprecated functionality in latest scipy, numpy, and sympy
+ Fix syntax for python 3.9

Removed

+ Support of python < 3.7, numpy < 1.16, scipy < 1.1, sympy < 1.3
(similar to the upcoming Kwant release)

Fixed
- Downgraded scipy version requirement to 0.19, to maintain
compatibility with Kwant
- Correctly set model format when reshaping models with sparse values

Added
- Added basic reference documentation
- Added citation instructions in CITING.md
- Added a tutorial with downloadable Python files and Jupyter notebooks

Fixed
- Allow 'display_family' to work when IPython is not installed
- Bug when multiplying a model by a sympy expression (undefined variable)
- Bug when multiplying BlochCoeff with sympy expression (sympy multiplication was
being used, when BlochCoeff multiplication should have been used)
- Remove copying of Sympy symbols (the hash values for the original and copy is
different in some cases (probably a bug in Sympy)), which broke comparison
between Models.
- Remove 'real' assumption from Sympy symbols in 'hamiltonian_from_family'; this
makes all Sympy symbols in Qsymm consistently have no assumptions.
- Minor clarifications to various docstrings

Removed
- Removed the notebooks from the source repository

Fixed
- Set correct content-type on long description metadata

Fixed
- Add long description to package metadata