Bump python requirement to 3.6

I've wanted to use f-strings for a long time, but avoided bumping the
python version requirement for fear it'd make it a challenge to use on
some enterprise distros.  However, it has been a while, and RHEL7 is
now old (uses python 3.6.8) as is Ubuntu focal (a.k.a. 20.04; uses
python 3.8.2) and Debian buster (uses python 3.7.3).  So, even
enterprise distributions from a few generations back have a new enough
python.  Bump the requirement now.

Signed-off-by: Elijah Newren <[email protected]>

Documentation/Contributing.md

@@ -34,7 +34,7 @@ with a few exceptions:
       2019 and should not be used.  (Commit 4d0264ab723c
       ("filter-repo: workaround python<2.7.9 exec bug", 2019-04-30)
       was the last version of filter-repo that worked with python2).
-    * You can depend on anything in python 3.5 or earlier.  I may bump
+    * You can depend on anything in python 3.6 or earlier.  I may bump
       this minimum version over time, but do want to generally work
       with the python3 version found in current enterprise Linux
       distributions.

README.md

@@ -40,7 +40,7 @@ filter-repo requires:
 
   * git >= 2.22.0 at a minimum; [some features](#upstream-improvements)
     require git >= 2.24.0 or later
-  * python3 >= 3.5
+  * python3 >= 3.6
 
 # How do I install it?
 

pyproject.toml

@@ -10,7 +10,6 @@ classifiers = [
     "Operating System :: OS Independent",
     "Programming Language :: Python",
     "License :: OSI Approved :: MIT License",
-    "Programming Language :: Python :: 3.5",
     "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
@@ -23,7 +22,7 @@ classifiers = [
     "Programming Language :: Python :: Implementation :: PyPy",
 ]
 license.text = "MIT"
-requires-python = ">= 3.5"
+requires-python = ">= 3.6"
 dynamic = ["version"]
 
 [project.urls]