Add PyQt calculator tutorial

pyqt-calculator-tutorial/pycalc/.gitignore

+# Byte-compiled / optimized
+__pycache__/
+*.py[cod]

pyqt-calculator-tutorial/pycalc/LICENSE.txt

+The MIT License (MIT)
+
+Copyright (c) 2014 Prahlad Yeri
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
\ No newline at end of file

pyqt-calculator-tutorial/pycalc/README.md

+# PyCalc
+
+PyCalc is a sample calculator implemented using Python 3, and with a [PyQt5](https://www.riverbankcomputing.com/static/Docs/PyQt5/introduction.html) GUI. PyCalc implements the most basic math operations, that is, division, multiplication, addition, and subtraction.
+
+PyCalc is intended to be a demonstrative example on how you can implement a Python + PyQt5 GUI application using the Model-View-Controller (MVC) pattern.
+
+## Screenshot
+
+![Screenshot](screenshot.png)
+
+## Requirements
+
+For PyCalc to work, you need to have a proper installation of [Python](https://www.python.org) >= 3.6. Then you need to install the PyQt5 library in your system. This can be done by using `pip3` as follows:
+
+```console
+$ sudo pip3 install pyqt5
+```
+
+We don't recommend you to install PyCalc's requirements directly into your system, as PyCalc is just a sample application. So, you can use a virtual environment to test PyCalc out. To do that, you can run the following commands:
+
+```console
+$ python3 -m venv pycalc
+$ source pycalc/bin/activate
+$ pip install pyqt5
+```
+
+After this steps are finished, you can run and test PyCalc as described in the next section.
+
+**Note**: For more information on how to install PyQt5, you can take a look at the related topic on the [project's documentation](https://www.riverbankcomputing.com/static/Docs/PyQt5/installation.html).
+
+## How to Run PyCalc
+
+To run PyCalc from your system's command-line and try it out, you can execute the following command:
+
+```console
+$ python3 pycalc.py
+```
+
+After running this command, you'll see PyCalc running on your screen.
+
+## How to Use PyCalc
+
+To use PyCalc, you just need to enter a valid math expression using your mouse, and then press `Enter` or click on the `=` sign:
+
+![Screenshot](pycalc-howto.gif)
+
+## About the Author
+
+My name is Leodanis Pozo Ramos. I'm a **freelance Python Developer and [technical writer](https://realpython.com/team/lpozoramos/)**. If you need more information about me, and my work, then you can take a look at my [personal page](https://lpozo.github.com/).
+
+## License
+
+PyCalc is released under the [MIT License](https://opensource.org/licenses/MIT).

pyqt-calculator-tutorial/pycalc/requirements.txt

+PyQt5==5.12.2