keep fake User-Agent header for all requests

Looks like some PRONOTE instances actively block clients that have
a weird User-Agent header. It should be fine to send it on all requests.

The header can be changed using:

    import pronotepy.pronoteAPI
    pronotepy.pronoteAPI.HEADERS = { "User-agent": "...", ... }

pronotepy/pronoteAPI.py

@@ -33,14 +33,21 @@
     25: "[ERROR 25] Exceeded max authorization requests. Please wait before retrying...",
 }
 
+HEADERS = {
+    "User-Agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:73.0) Gecko/20100101 Firefox/73.0",
+}
+
 
 class _Communication(object):
     def __init__(
         self, site: str, cookies: Optional["RequestsCookieJar"], client: ClientBase
     ) -> None:
         """Handles all communication with the PRONOTE servers"""
         self.root_site, self.html_page = self.get_root_address(site)
+
         self.session = requests.Session()
+        self.session.headers.update(HEADERS)
+
         self.encryption = _Encryption()
         self.attributes: dict = {}
         self.request_number = 1
@@ -57,11 +64,6 @@ def initialise(self) -> Tuple[Any, Any]:
         Initialisation of the communication. Sets up the encryption and sends the IV for AES to PRONOTE.
         From this point, everything is encrypted with the communicated IV.
         """
-        # some headers to be real
-        headers = {
-            "connection": "keep-alive",
-            "User-Agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:73.0) Gecko/20100101 Firefox/73.0",
-        }
 
         # get rsa keys and session id, retry 3 times
         for _ in range(3):
@@ -70,7 +72,6 @@ def initialise(self) -> Tuple[Any, Any]:
                 get_response = self.session.request(
                     "GET",
                     f"{self.root_site}/{self.html_page}",
-                    headers=headers,
                     cookies=self.cookies,
                 )
                 self.attributes = self._parse_html(get_response.content)