Supplement the current requests-based HTTP backend by a new one based on HTTPx

From the changelog:

 * The requests-based HTTP backend has been supplemented by another backend based on [HTTPx](https://www.python-httpx.org/) for Python 3.6+
    * Due to a minor limitation within the library (no ability to apply address family restrictions during name resolution) this currently included as a preview and must be manually enabled, to do this ensure that the `httpx` library is installed in your Python environment and run your program with the environment variable *PY_IPFS_HTTP_CLIENT_PREFER_HTTPX* set to *yes*.
    * In the hopefully not too long future, HTTPx will be used to finally provide async/await support for this library.

Additionally this brings of `http*.py` things to 100% and total coverage to ~92%!

Author: ntninja

.travis.yml

@@ -12,6 +12,8 @@ matrix:
       - python: "pypy3"
       - python: "3.8"
         env: IPFS_VERSION=compat
+      - python: "3.8"
+        env: TOXENV=py3-httpx
       - python: "3.8"
         env: TOXENV=styleck
         before_install: ":"

CHANGELOG.md

@@ -18,6 +18,9 @@ Other changes:
     * The new *period_special* parameter allows toggling whether glob patterns match dot-files implicietly and defaults to `True` (previously it was effectively `False`)
     * The new *follow_symlinks* parameter similarily determines whether symbolic links will be followed when scanning directory trees and defaults to `False` (the previous default on Unix, albeit this likely wasn't intentional)
     * `.add()` will now limit its scan to the directories required to match the given glob patterns (passing in regular expression objects will still scan the tree unconditionally however) – custom matchers have full control over which directories are visited
+ * The requests-based HTTP backend has been supplemented by another backend based on [HTTPx](https://www.python-httpx.org/) for Python 3.6+
+    * Due to a minor limitation within the library (no ability to apply address family restrictions during name resolution) this currently included as a preview and must be manually enabled, to do this ensure that the `httpx` library is installed in your Python environment and run your program with the environment variable *PY_IPFS_HTTP_CLIENT_PREFER_HTTPX* set to *yes*.
+    * In the hopefully not too long future, HTTPx will be used to finally provide async/await support for this library.
 
 py-ipfs-http-client 0.4.12 (21.05.2019)
 ---------------------------------------

ipfshttpclient/client/base.py

@@ -86,10 +86,10 @@ def ident(value: T) -> T:
 
 class ResponseWrapIterator(ty.Generic[T, R]):
 	__slots__ = ("_inner", "_item_wrap_cb")
-	#_inner: http.StreamDecodeIterator
+	#_inner: http.StreamDecodeIteratorSync
 	#_item_wrap_cb: wrap_cb_t
 
-	def __init__(self, inner: http.StreamDecodeIterator, item_wrap_cb: wrap_cb_t):
+	def __init__(self, inner: http.StreamDecodeIteratorSync, item_wrap_cb: wrap_cb_t):
 		self._inner = inner
 		self._item_wrap_cb = item_wrap_cb
 
@@ -277,7 +277,7 @@ def __init__(
 		if auth is None and (username or password):
 			auth = (username, password)
 
-		self._client = http.HTTPClient(
+		self._client = http.ClientSync(
 			addr, base, offline=offline,
 			auth=auth, cookies=cookies, headers=headers, timeout=timeout,
 		)