doc - Update required Python version to 3.7+

- Required Python version was 3.6.2
- Dicts are declared as ordered since 3.7

Author: phibos

README.rst

@@ -33,7 +33,7 @@ Install
 
 Supported Python versions:
 
-* Python >= 3.6.2
+* Python >= 3.7
 * PyPy3
 
 **Install:**

docs/source/introduction.rst

@@ -6,7 +6,7 @@ Requirements
 
 Supported Python versions:
 
-* Python 3.6+
+* Python 3.7+
 * PyPy3
 
 Installation

setup.cfg

@@ -8,7 +8,6 @@ classifiers =
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
@@ -20,7 +19,7 @@ project_urls =
     Issue Tracker = https://github.com/DinoTools/python-overpy/issues
 
 [options]
-python_requires = >=3.6
+python_requires = >=3.7
 include_package_data = true
 zip_safe = false