docs: drop support for python3.7

Author: long2ice

README.rst

@@ -29,7 +29,7 @@ You can find the docs at `Documentation <https://tortoise.github.io>`_
    Tortoise ORM is a young project and breaking changes are to be expected.
    We keep a `Changelog <https://tortoise.github.io/CHANGELOG.html>`_ and it will have possible breakage clearly documented.
 
-Tortoise ORM is supported on CPython >= 3.7 for SQLite, MySQL and PostgreSQL and Microsoft SQL Server and Oracle.
+Tortoise ORM is supported on CPython >= 3.8 for SQLite, MySQL and PostgreSQL and Microsoft SQL Server and Oracle.
 
 Why was Tortoise ORM built?
 ---------------------------

docs/CONTRIBUTING.rst

@@ -99,7 +99,7 @@ Tortoise ORM follows a the following agreed upon style:
 * Always try to separate out terms clearly rather than concatenate words directly:
     * ``some_purpose`` instead of ``somepurpose``
     * ``SomePurpose`` instead of ``Somepurpose``
-* Keep in mind the targeted Python versions of ``>=3.7``:
+* Keep in mind the targeted Python versions of ``>=3.8``:
     * Do use f-strings
 * Please try and provide type annotations where you can, it will improve auto-completion in editors, and better static analysis.
 

docs/index.rst

@@ -13,7 +13,7 @@ It's engraved in it's design that you are working not with just tables, you work
 
 Source & issue trackers are available at `<https://github.com/tortoise/tortoise-orm/>`_
 
-Tortoise ORM is supported on CPython >= 3.7 for SQLite, MySQL and PostgreSQL.
+Tortoise ORM is supported on CPython >= 3.8 for SQLite, MySQL and PostgreSQL.
 
 Introduction
 ============