diff --git a/.python-version b/.python-version
index c49282585a..8b7b0b52e5 100644
--- a/.python-version
+++ b/.python-version
@@ -1 +1 @@
-3.5.6
+3.6.12
diff --git a/.travis.yml b/.travis.yml
index de92303e55..95e0853d01 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -204,7 +204,7 @@ after_success:
       env:
       cache: false
       language: python
-      python: '3.5' # Oldest supported version according to doc/dependencies.md
+      python: '3.6' # Oldest supported version according to doc/dependencies.md
       install:
         - set -o errexit; source ./ci/lint/04_install.sh
       before_script:
diff --git a/configure.ac b/configure.ac
index c870efe3f7..4f5f93771d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -99,8 +99,8 @@ AC_PATH_TOOL(RANLIB, ranlib)
 AC_PATH_TOOL(STRIP, strip)
 AC_PATH_TOOL(GCOV, gcov)
 AC_PATH_PROG(LCOV, lcov)
-dnl Python 3.5 is specified in .python-version and should be used if available, see doc/dependencies.md
-AC_PATH_PROGS([PYTHON], [python3.5 python3.6 python3.7 python3.8 python3 python])
+dnl Python 3.6 is specified in .python-version and should be used if available, see doc/dependencies.md
+AC_PATH_PROGS([PYTHON], [python3.6 python3.7 python3.8 python3.9 python3 python])
 AC_PATH_PROG(GENHTML, genhtml)
 AC_PATH_PROG([GIT], [git])
 AC_PATH_PROG(CCACHE,ccache)
diff --git a/doc/dependencies.md b/doc/dependencies.md
index 8869b93fee..1576a65461 100644
--- a/doc/dependencies.md
+++ b/doc/dependencies.md
@@ -20,7 +20,7 @@ These are the dependencies currently used by Dash Core. You can find instruction
 | librsvg | |  |  |  |  |
 | MiniUPnPc | [2.2.2](https://miniupnp.tuxfamily.org/files) |  | No |  |  |
 | PCRE |  |  |  |  | [Yes](https://github.com/dashpay/dash/blob/develop/depends/packages/qt.mk) |
-| Python (tests) |  | [3.5](https://www.python.org/downloads) |  |  |  |
+| Python (tests) |  | [3.6](https://www.python.org/downloads) |  |  |  |
 | qrencode | [3.4.4](https://fukuchi.org/works/qrencode) |  | No |  |  |
 | Qt | [5.12.11](https://download.qt.io/official_releases/qt/) | [5.5.1](https://github.com/bitcoin/bitcoin/issues/13478) | No |  |  |
 | SQLite | [3.32.1](https://sqlite.org/download.html) | [3.7.17](https://github.com/bitcoin/bitcoin/pull/19077) |  |  |  |
diff --git a/test/functional/README.md b/test/functional/README.md
index 1f3d20288f..155ef1bbe9 100644
--- a/test/functional/README.md
+++ b/test/functional/README.md
@@ -32,7 +32,7 @@ don't have test cases for.
 - When subclassing the BitcoinTestFramwork, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
-- Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
+- Use `f'{x}'` for string formatting in preference to `'{}'.format(x)` or `'%s' % x`.
 
 #### Naming guidelines
 
diff --git a/test/functional/feature_block.py b/test/functional/feature_block.py
index 44dc80ccdb..c41bd5a10f 100755
--- a/test/functional/feature_block.py
+++ b/test/functional/feature_block.py
@@ -123,7 +123,7 @@ class FullBlockTest(BitcoinTestFramework):
         # Allow the block to mature
         blocks = []
         for i in range(NUM_BUFFER_BLOCKS_TO_GENERATE):
-            blocks.append(self.next_block("maturitybuffer.{}".format(i)))
+            blocks.append(self.next_block(f"maturitybuffer.{i}"))
             self.save_spendable_output()
         self.send_blocks(blocks)
 
@@ -161,8 +161,8 @@ class FullBlockTest(BitcoinTestFramework):
             if template.valid_in_block:
                 continue
 
-            self.log.info("Reject block with invalid tx: %s", TxTemplate.__name__)
-            blockname = "for_invalid.%s" % TxTemplate.__name__
+            self.log.info(f"Reject block with invalid tx: {TxTemplate.__name__}")
+            blockname = f"for_invalid.{TxTemplate.__name__}"
             badblock = self.next_block(blockname)
             badtx = template.get_tx()
             self.sign_tx(badtx, attempt_spend_tx)
@@ -1364,12 +1364,12 @@ class FullBlockTest(BitcoinTestFramework):
 
     # save the current tip so it can be spent by a later block
     def save_spendable_output(self):
-        self.log.debug("saving spendable output %s" % self.tip.vtx[0])
+        self.log.debug(f"saving spendable output {self.tip.vtx[0]}")
         self.spendable_outputs.append(self.tip)
 
     # get an output that we previously marked as spendable
     def get_spendable_output(self):
-        self.log.debug("getting spendable output %s" % self.spendable_outputs[0].vtx[0])
+        self.log.debug(f"getting spendable output {self.spendable_outputs[0].vtx[0]}")
         return self.spendable_outputs.pop(0).vtx[0]
 
     # move the tip back to a previous block