Styleframe

No longer relying on openpyxl's colors definition. Related to [GitHub issue 73](https://github.com/DeepSpace2/StyleFrame/issues/73)

Hotfix release - setting maximum versions for dependencies. Related to [GitHub issue 73](https://github.com/DeepSpace2/StyleFrame/issues/73)

* **Removed Python 2.7 support**
* **Added Python 3.8 support**
* Renamed package name to `styleframe` (all lowercase) in accordance of PEP8
* Added `.style` accessor. This allows for easy selection/indexing based on style, for example:
`sf.loc[sf['col_name'].style.bg_color == utils.colors.yellow]`

or

`sf.loc[~sf['col_name'].style.bold]`

* Added `default_grid` to `utils.borders` to allow usage of the default spreadsheet grid
* Added `read_excel_as_template` method
* Fixed a bug that prevented saving if `read_excel` was used with `use_openpxl_style=True`, see [GitHub issue 67](https://github.com/DeepSpace2/StyleFrame/issues/67)
* Allowing usage of pathlib.Path in `to_excel`, see [GitHub issue 69](https://github.com/DeepSpace2/StyleFrame/issues/69)
* Added ability to execute the tests from the commandline: `styleframe --test`

* `style_alternate_rows` can accept all arguments that `apply_style_by_indexes` accepts as kwargs.
* Added `cols_to_style` argument to `apply_headers_style`

* Fixed a bug that caused `apply_style_by_indexes` not to work in case the dataframe had non-integer indexes in some cases
* Added support for text rotation

* Fixing pandas dependency for different Python versions, related to 52, 53